What are your opinions on which is the best ascended power and why?

I'm voting Psychokinesis. I think its the most versatile power tree with a wide variety of uses. The first power is really quite a gem, allowing the psyker to fly at incredible speeds or fling a charged melta bomb at an enemy(albeit on a BS test). The last power could potentially shutdown or at least tarpit an entire army.

Well, as with most things it really depends on your personal play style; though I have to agree that the Psychokinesis tree is a spectacular one.

Despite my current character being a rather potent Pyromancer I passed up the Burning Apocalypse tree (I already have Holocaust - why bother with anything else?) in favor of Witchesbane. Purgatus overbleed can have such interested consequences.

Daemonsbane also tempted me sorely, however I play Hereticus primarily and have already invested in the Pure Faith talents so banishing is no longer a problem should I come across anything possessed.

All of them are pretty mighty and awesome, but I am going to have to go with "WILL UNLEASHED" for all it's yummy Ravenor-esque goodness.

When a death culy assassin can can charge and make 3 melee attacks combined with unatural agility and other initiative bonuses the big nasty is usually dead before anyone even gets to say " my turn " .