This is a follow-up to my Librarians, why not? article. Each Librarian will list the powers I find most attractive and why. Most libbies can only choose 2 powers, but the GK one can buy as many as he wants.
Space Marine Librarian
Null Zone, Avenger, Gate of Infinity
Libby, TDA w/ SS (Null Zone, Gate of Infinity) leading 5x TH/SS Termies in a LRC
Libby, (Avenger, Gate of Infinity), Drop Pod, 10x Sternguard w/ Combi-weapons
The weakest Librarian out of the bunch, the standard SM Librarian gets access to quite a few unique powers. Null Zone really screws anything with invul saves; ranging from TH/SS Terminators, Wyches in CC or 2++ Shadow Fields. Avenger is one of the best templates in the game as S5 AP3 cuts through MEQ like butter. GoI is a little tricky but can play a big part in your army as you can take your TH/SS Termies from one location to another or carry a large bunch of Sternguard into your enemies' face.
Blood Angels Librarian
Unleash Rage, Sanguine Sword, Shield of Sanguinius, Blood Lance
Libby, TDA w/ SS (Unleash Rage, Sanguine Sword) leading 5x Assault Termies in a LRC
Libby, JP (Unleash Rage, Shield of Sanguinius) leading 10x ASM w/ PF, 2x Meltaguns
The Blood Angels Librarian holds many destructive abilities that compliment the offensive nature of the BA army. Unleash Rage is one of the reasons the Reclusiarch is a subpar choice. Preferred Enemy for multiple rounds of combat is much stronger than one round of re-roll hits. Sanguine Sword allows the Librarian to take on anything in the game because S10 can touch anything on 2s while punching through the strongest armor. Shield allows BA to spam mech like no other while advancing towards the enemy. Then, there's the Blood Lance. It's not as strong as JotWW in terms of what it can target, but it can threaten any vehicle with its S8 AP1 Lance. Versus enemy tank lines, if you can line up a good shot, you might be able to take out quite a few targets per turn. The best thing about these abilities is that both the offensive assault ones I noted (UR and SS) can be used in either player's turn. Shield is a defensive ability that fires during your enemy's shooting phase and Blood Lance is in your shooting phase. This healthy mix of choices allows the BA Librarian to get off psychic powers in either player's turn.
Grey Knights Librarian
Shrouding, Sanctuary, Might of Titans, Warp Rift
Libby, (Shrouding, Sanctuary, Might), 3x Servo-skulls, MC Halberd leading..
6x Paladins (Apoth, MC DH, 2x MC Psycannons, BBanner, MC, all Halberds)
10x GKT (MC DH, DH, rest Halberds, 2x Psycannons, BBanner, Psybolts)
Some say the GK Libby isn't needed, but I think he's one of the best buys in the book. For 150 points, you get a Epistolary that's in Terminator Armor, can have a 4++ in close combat because of his Force Sword, has access to I6 Halberds or MC weapons, and is equipped with Frag and Krak grenades. On top of this, he has 4 amazing psychic powers that compliments the GK army perfectly. Shrouding keeps your units safe in cover with a whopping 3++ save, Sanctuary provides assault protection that covers a large area, and Might of Titans allows your units to punch through tanks with +1S and 2d6 armor pen. This can be used in conjunction with Hammerhand for S6 vs. T4 MEQ or Hive Tyrant strength vs. armor. Lastly, we have Warp Rift. I like this ability because it's essentially a template-sized JotWW. Sure, it doesn't target models, but against slow, expensive units who are bunched up (think Nids and Orks), this ability is invaluable. Just like the BA Librarian, the GK Libby can get off multiple abilities per turn since Shrouding is cast in the enemies' shooting phase, Sanctuary in his assault, Might in your assault, Hammerhand whenever and Warp in your shooting. An unupgraded GK Libby can cast 2 abilities per player turn, and up to 4 per game turn. With Ld.10, he is my go-to guy to get off Hammerhand when my Paladins use their psychic ability to activate their Force Weapons via Brotherhood Banner. Instant win. Oh, and he has access to Servo-skulls, which is pretty much a must-have if you intend on taking Deep Strikers.
Space Wolves Rune Priest
Murderous Hurricane, Living Lightning, Jaws of the World Wolf
Rune Priest (Chooser, Murderous Hurricane, LL) leading..
8x Grey Hunters w/ WG (Rhino, PF/cbM, Meltagun, WS, MotW)
Last, but certainly not least, is the SW Rune Priest. The best thing about the Rune Priest is that his runic weapon nullifies enemy psychic abilities on a 4+. No need to roll off like the other libbies, a 4+ within 24" is all you need to shut down enemy powers. In addition, he can take quite a few pieces of wargear to make him stand out more so than the other librarians. First, he can take Wolftooth Necklace that makes him hit anything with a WS at 3+, no matter what. It doesn't matter if he's fighting a Gaunt or a Bloodthirster, if you roll a 3+, you hit. See a trend here? Second, is his ability to take Runic Armor, which is the SM-equiv to Artificer Armor w/ a built-in psychic protection. Third, is his ability to take Sagas that improves his killing potential ontop of his natural Counter-attack. He has so many different wargear options that he makes other librarians look broke ass poor, save maybe the GK one. I guess since the Chooser of the Slain is pretty much an auto-include for me, he should get a mention. 18" perched on top of a high point gives the RP +1BS with Living Lightning and pushes back all infiltrated units. Viewed as the best HQ choice in the game by many players, I take him in every game with Space Wolves.
As for powers, I find Murderous Hurricane, Living Lightning and Jaws to be the best choices. Murderous Hurricane allows you to slow down and destroy enemy infantry, Living Lightning allows you to punch through armor from infinite range, and Jaws is just... jaws. No other psychic ability in the game caused as much ruckus as Jaws did when it first came out and it's still being talked about today. It has a 24" line that can snipe any model it touches and if they fail a I test, they're removed from play. Laff.