For Dark, most of the powers are good.
The hold is skippable but useful( if you don't want a hold ). Black Hole probably is too, I'm not planning on getting it anyway.
The res doubles up as a good 15-20 second disorient, so can be used as extra control.
Tar Patch and Fearsome Stare are IMO must haves. Darkest Night and Twilight Grasp are too.
Shadowfall is more skippable but it is still a very good power for the E/N resistance and group stealth. In CoV and PvP the Fear/Confuse protection is also great.
@Unthing ... Mostly on Union.
So any help on the Psi side of things?
Take the whole lot 
Failing that, I'd drop, in this order: Scramble Thoughts, Psychic scream, Psionic Lance, Psionic Tornado, Psychic Wail, Subdue, Will domination, TK Blast.
As for slotting (Assuming stamina and hasten):
Mental Blast: Nothing
Subdue: 1 acc 3 dam
Psionic Lance: 1 acc 3 dam 1 EndRdx 1 Rech/IntRdx
Psychic Scream: 1 acc 3 dam 1 End 1 Rech
TK Blast: 1 acc 3 dam 0-1 rech 0-1 EndRdx
Will Domination: 1 acc 3 dam 1-2 rech 0-1 EndRdx 0-1 Sleep
Psionic Tornade: 1 acc 3 dam 1 rech 1 End
Scramble Thoughts: 1 acc 0-3 Stun 0-2 recharge 0-1 End
Psychic Wail:0-1 Acc 3 Dam 0-3 rech (Possibly stun?)
Shadowfall is invaluable in the later levels as it provides much (much, much, much) desired psi resistance.
To emphasise this point, when fighting psi mobs, you should always refer to Inv tanks as Mr Squishy-head, and tell them to stand near you to save them from the nasty psychic clockwork.
Dead Calm's Defender Manifesto
Ive decided to roll one of these as ive quite liked Dark when playing a MM. Ive taken Psionic Blast as its unque to defenders.
As ive never played a defender before (ive played everything else except a stalker) id like some advice on:
a)Do the sets go well together?
b)If they do, what powers are the must haves?
No need for technical jargon such as percentages btw
Thanks for any responses guys n gals