1 – Gates of Infinity — Great for getting Abbadon and those Terminators accros the board turn 1.
2 – Hammerhand — Adding +2S to your unit is amazing, no matter who they are; even basic CSM attacking at S6 are quite solid, not to mention if you buff them with Mark of Khorne or Mark of Nurgle, etc.
3 – Sanctuary — Combine with a unit that has Mark of Tzeentch and a decent Invul save already, like Terminators or 1K Sons, for a 3++ save.
4 – Purge Soul — Decent little Focused Witchfire, especially on a PML (4) like Ahriman, who can spit out multiple witchfire a turn, in addition to normal shooting.
5 – Cleansing Flame — Amazing anti-horde power… enough said.
6 – Vortex of Doom — Ultimate “Nuclear Strike” witchfire power; high risk, high reward.
Now all of these are options, but one that I like the best is taking a Chaos Lord on a Bike with Blade of the Relentless (from Crimson Slaughter book) and a bunch of Chaos Spawn and attaching a Biker Sorceror with Hammerhand to them… that means that the Chaos Lord is Striking at S6, enabling him to power up his sword quickly with lots of kills (he might even get 4-5 kills in a single round, making his weapon S+2 AP2 naturally… combine this with Hammerhand again and you are striking at S8 AP2, all at I5). It also means that the Spawn are S7, so they can threaten vehicles and infantry alike.
Finally, the Sanctic powers can combine well with Chaos Marks, like the above mentioned Chaos Lord… give him the 2+ armor from Crimson Slaughter supplement, Sigil of Corruption, and Mark of Nurgle, then get Sanctuary and Hammerhand off and you have Biker Lord with T6, 2+/3++ save, and a S6 AP3 sword that gets better the more enemies it kills. Alternately, give him and a Chaos Biker Escort Mark of Slaanesh (for +1I, so he is now T5, 2+/3++ with I6) and the Tzeentch Banner for FNP and he becomes both more survivable and a threat to even the top tier combat characters.