Space Marines

Oath of Moment : In your command phase choose one enemy unit in your opponents army.  Until your next command phase your units get to re-roll all hit rolls against this target enemy unit.  A terrific army rule that allows you to focus on the biggest threats one at a time and make sure your force is able to hit it maximum efficiency. 

Space Marine Chapters :  This is the generic 'You can only have a maximum of ONE second faction keyword in your army at a time'.  For example, it stops you from playing with both the Dark Angels Primarch Lion El' Johnson while also bringing Calgar from the Ultramarines in the same army.  NO SOUP FOR YOU!  (which is fine, no one wants to go back to the early days of 8th edition ever again).

Detachment Rules : Extremis-level Threat

A detachment rule that is only able to be used once per battle better be good and honestly this one is pretty damn good.  Once per game when you pick your Oath of Moment target you can activate this ability to give your units Re-roll your wound rolls as well... that is one hell of an upgrade.  

Enhancements : 

The Imperium's Sword - This is an OK enough enhancement, giving the bearer +1 to the attack characteristic of each melee weapon equipped by the bearer.  More interesting is once per game you can activate its super power and give all of the melee weapons bearer's unit +1 attack as well.  It's always nice when your character can share his Enhancement with the unit he is attached to, but +1 Attack isn't THAT big of a deal when other armies like Blood Angels can just get that and then some on the charge.

Fear Made Manifest (AURA) - This is another one of those 'If battle-shock mattered this could be cool... too bad it doesn't.' enhancements.  When an enemy unit within 6" of the bearer fails a battle-shock check  one model is destroyed chosen by its controlling player.  Also once per game (so the one time this actually may happen) you can choose to destroy D3 models in this way instead... which means your going to roll a 1 and it was all for nothing.  HARD PASS.  I would rather just not spend the points.

Rites of War - This one increases the bearers OC by 1 which is useful enough.  More interesting is once per game at the start of any phase you can increase the OC of every model in the unit by 1 as well.  This is good for making sure you can keep an objective under your control or help you flip an opponents objective to prevent them from scoring points, so it is good stuff for sure. 

Iron Resolve - A nice enhancement that gives the bearer a 5+ FNP which is always nice to have.  It also allows you to once per game when the bearers unit is the target of an attack to activate this and give the unit a 5+ FNP as well which is very useful.   

Detachment Rules : Shield of the Imperium

Our Imperial Fist detachment, this detachment has some really interesting play for pretty much any unit in the army giving every model in your army the [Heavy] keyword.  What is better is the fact that if the model already has the [Heavy] keyword then each time the model remains stationary and fires then you get to add +1 to the wound roll as well.   I have played this detachment a few times and while I definitely think there is some play here the problem is the detachment really wants to be standing still to take advantage of its rules and that some times that doesn't help you win games.

Enhancements : 

Indomitable Fury - This allows your Gravis Character to get back up 'Randy Marsh' style on a 2+ after they are killed.  This can really be a useful ability but it really depends on WHEN your Gravis Character is killed to determine how good it really is.  Your Gravis Captain hasn't heard no bell... 

Fleet Commander - Ah, the good old Imperial Fist 'Orbital Bombardment' ability from days gone is back again.  This is honestly a pretty useful ability to have even if it is once per game.  At the start of your shooting phase pick a point on the table and put a marker there.  Then at the start of your NEXT shooting phase pick another point 12" away from the first marker.  Any enemy units that are between these two points on a dice roll of 3+ take D3 mortal wounds, not a bad way to try and scare an enemy off of an objective. 

Stoic Defender - An OK Enhancement, this gives the bearer's unit a 6+ FNP while the unit is within range of an objective marker you control.  This also helps you keep objectives under your control as the bearers unit will halve their OC if they become battle shocked instead of having it turned to 0.  Not a bad Enhancement but not something all that exciting as well.  

Architect of War - This Enhancement allows the bearers unit's ranged weapons to gain the [Ignores Cover] ability.  Considering how easy it is to get cover in this edition this is a pretty useful ability to grab.  

Detachment Rules : Close-range Eradication

The Salamanders Detachment, this one gives all models in your army the [Assault] ability.  Also, each time an attack is made with a ranged weapon that targets an enemy unit within 12" you get to add +1 to the strength of that attack.  This detachment is highly underrated and is one of my favorite ones in the book (also Salamanders are one of my favorite Chapters of all time).  

Enhancements : 

Champion of Humanity - While the bearer is leading a unit, models in that unit can ignore any or all modifiers to their characteristics and/or to any roll or test made for them (excluding modifiers to saving throws).  THIS IS ACTUALLY HUGE.  This shuts down enemy units that have abilities like -1 Damage as well as preventing -1 to Wound, -1 to Hit, all those pesky 'protective' abilities some units have that make them so annoying to deal with.  You have to be happy when you have access to this type of ability.  

War-tempered Artifice - Now this is the type of Enhancement I like, simple and to the point.  Add 3 to the strength characteristics of the bearers melee weapons?  Why yes, I have been working out at the Gym, I can now bench press a Str 11 Power Fist like it's nothing.  

Forged in Battle - Another great Enhancement, once per TURN you can turn a hit or save roll you already rolled into  an unmodified 6.  Personally I would use it on the hit rolls until the bearer is in danger of being able to be actually wounded then save the ability to keep yourself alive with always being able to turn a failed save into an auto pass.  

Adamantine Mantle - This one, while funny and potentially really annoying for your opponent to deal with, isn't really the best option here.  It reduces all damage done to the bearer by 1 which is cool.  What is funny is that GW's math currently works like this - Multiply / Divide / Add / Subtract.  That means if you have, say, a Gravis Captain who already reduces damage by half this would reduce by half then subtract 1.  Unfortunately this won't matter one bit when the Captain is getting blasted by 1000 las gun shots by an angry platoon of guardsmen, but man, you sure could survive getting punched in the face by Canis Rex at least.... 

Detachment Rules : Combat Doctrines

This detachment used to be the Index detachment and is easily recognized as the Detachment home for the Ultramarines.  Taking advantage of Combat Doctrines, this detachment allows you in your Command Phase to choose one of 3 Doctrines to be active for your force until your next Command Phase. Devastator Doctrine allows you to Advance and Shoot, Tactical Doctrine allows you to shoot and declare a charge in a turn in which you do a Fall Back move, and Assault Doctrine allows you to declare a charge in a turn in which you Advance.  Honestly this detachment is easily considered one of the strongest in the Codex and it is easy to understand why.  

Enhancements : 

Artificer Armour - This gives the bearer a 5+ FNP as well as a 2+ save which is nice.  It's also pretty cheap points wise so if you have the extra points left over there are worst things to consider taking than this.  

The Honour Vehement - This is your 'take a melee character and turn him up to 11' enhancement.  It adds 1 to the attack and strength characteristics of the bearers melee weapons, adding 2 if you are in the Assault Doctrine.   Very nice.

Adept of the Codex - This is a pretty useful Enhancement only held back by the fact it can only go on a Captain model.  It allows you to choose to activate the Tactical Doctrine instead of a different Doctrine, but only for the Captain's unit.  This seems cool, but the way it is worded means you are activating the Tactical Doctrine for the Captain's unit INSTEAD of activating a doctrine for your entire army meaning it's only going to come up twice a game probably.   

Fire Discipline - This Enhancement is a problem, you can tell by the fact that GW has priced it so absurdly high.  It grants the bearers unit [Sustained Hits 1] on their ranged weapons, and if your under the Devastator Doctrine you also Critical on a 5+.  That is an insane power boost for a units ranged potential, going even further using stratagems to always keep his unit in the Devastator Doctrine, add a Lieutenant in to also give the unit [Lethal Hits], and you can see how this could easily get out of hand and become a problem.  Having said that it IS incredibly powerful and if your taking this Detachment your probably going to always find a way to take this as well.

Detachment Rules : Armoured Wrath

This is the Iron Hands Detachment and is a pretty effective detachment if your looking to run a lot of MSU's that have powerful weapons that are low on the number of shots but high on the amount of damage they can do.  It allows each unit in the Detachment to re-roll a single hit, wound, OR damage roll each phase.  

Enhancements : 

Target Augury Web - This is a pretty useful Enhancement, it allows a Techmarine to pick a unit within 6" of the bearer in the Command Phase to gain [Lethal Hits] until your next Command Phase.  This may seem like nothing much, but keep in mind some vehicles like, say, the Repulsor, have a TON of small arms fire and [Lethal Hits] make these small weapons very dangerous. 

The Flesh is Weak - This is a very simple option to pick, giving the bearer a 4+ FNP.  Mathematically that's basically a 50% Increase to the bearers wound count and is pretty good on a unit like a Gravis Captain.

Adept of the Omnissiah - A VERY powerful Enhancement, granting the Techmarine bearer the ability to once per battle round, when a saving throw is failed for a friendly Vehicle within 6" of the Techmarine is failed, you can change the damage characteristic of that attack to 0.  This can make all the difference between a valuable Vehicle being blown away and surviving for another round so the Techmarine can get to repairing it.    

Master of Machine War - Another valuable Enhancement, this one allows the bearer to select one Vehicle within 6" of the bearer in the Command phase to gain the ability to Shoot even if it Fall's Back or Advances.  While Vehicles can already shoot while in engagement range with enemy units the ability to Fall back and still shoot is quite valuable and shouldn't be ignored.  

Detachment Rules : Lightning Assault

The White Scar's Detachment, this one has always bothered me.  It gives all units in the army have the ability to Advance or Fall Back and still Charge, which is quite good.  Unfortunately most of this detachment's stratagem's and enhancement's are focused around 'Mounted' units... so Chaplains on Bikes and Outriders unless your a Space Wolf player... that's cool I guess.

Enhancements : 

Fury of the Storm - This is the 'Make a character better at melee' enhancement and for the most part it succeeds.  Chaplains are pretty good melee characters, with their only real weakness being their AP being only -1.  This enhancement increases the Strength and AP of the bearer by 1, or by 2 if the bearer charged in this turn which is nice enough I suppose, but it isn't THAT big of a deal.

Portents of Wisdom - This is an OK enhancement, giving the bearer the ability to re-roll advance rolls while they are leading a unit.  This is pretty nice because it can be taken by any Adeptus Astartes character. 

Feinting Withdraw - Another mediocre enhancement, this one grants the bearer the ability to shoot in a turn in which it fell back which is OK.  It would be more useful if you could still charge after falling back as well but as it stands it's not all that useful unless you have a character attached to a unit that is focused on ranged firepower.  

Hunters Instincts - This is the Enhancement that is almost a guaranteed pick when you take this detachment.  It allows the bearer's unit to treat the current battle round as being one higher than it actually is for coming in from Strategic Reserve.  The down side is again this one is only allowed to be taken by a Mounted character which means Outriders and a Chaplain on bike... which isn't really a threatening unit all things considered.  Don't get me wrong, I love Outriders, but they are a tactical unit used for grabbing objectives early game and harassing flanking units.  Now if your playing Space Wolves then suddenly this is an amazing option because you have Thunderwolf Calvery meaning this becomes seriously powerful. 

Detachment Rules : Shadow Masters

The Ravenguard Detachment, this is a pretty useful detachment if your looking to gain some defensive buffs for your force as well as giving you some great movement shenanigans.  This detachment gives all units in your force the benefit of cover from ranged attacks as well as -1 to hit if the attacking model is more than 12" away from your unit which is cool.  

Enhancements : 

The Blade Driven Deep - An incredibly useful enhancement, this allows the bearers unit to gain the Infiltrators ability.  Dropping a big brick of 10 Terminators right in the center of the table before the game even starts is guaranteed to make your opponent think twice about how they plan to deploy.

Ghostweave Cloak - This is another good enhancement, giving the bearer the Lone Operative and Stealth abilities.  While some unit's won't find having Lone Operative to be that useful others will find it incredibly useful.  

Execute and Redeploy - This can only be taken on a Phobos model, but man is it a useful ability for sure.  In your shooting phase after the bearers unit has shot if they are not in engagement range of any enemy units the bearers unit can make a Normal move of up to 6".  If they do they can't charge in the same turn but who cares?  Take a Phobos Librarian, put them in a squad of 3 Eliminators and now you have a unit that can Move-Shoot-Move and still take all 3 of the good weapons that Eliminators can take.  What's not to love about that?

Shadow War Veteran - Another pretty useful ability, this one again can only go onto a Phobos unit but it does grant one heck of an Aura ability to that character.  They gain "Lord of Deceit", an aura that causes your opponent to spend an additional CP any time they try to use a stratagem that is targeting a unit within 12" of the bearer.  Take a squad of Reivers (here me out), put a Lieutenant in Phobos Armor in there with them and give him this enhancement.  Now you have a cheap harassment unit that can deep strike in, shoot and then move closer to a back field unit, and now you have yourself an annoying flea of a unit just sitting back there causing chaos.