There are a lot of situational variables here but if i had to pick one really great advantage thats just all-around excellent i would have to go with "Gravitational Polarity" for the Force powerset's Force eruption.

giving yourself a 20% damage boost for almost no end cost at any time on an already handy power for the bargain price of 1 advantage point is a really great choice... generally speaking of course.

I've fallen in love with Condemn > Redemption Denied. Every target hit by condemn triggers an AoE damage when the stun breaks, even if they werent technically held by Condemn.

Those are good, but I would have to go with either force shield/sheath, Diffraction of sound on sonic blaster or Lightning Sheathe body matter whatever advantage for strong unbroken not OP advantages, of course the most OP advantages are on Lash and Grasping? Shadows.

I'll go with the lightning sheath one if have to pick one, + any energy builder and you are essentially invulnerable for the duration of the energy form.

O and then there's Swallower's tail cut.

probably the ego surge procing advantage for Kinetic darts.

Fatal Allure.

Frantic Blasting or whatever concussive blast's snaring advantage is.

Id have to say either Nimble Mind on Ego Surge or the AoE stun on the Sonic Arrow.

I haven't tested it yet, but the Reanimator advantage for Resurrection Serum could be very useful, it's definitely the most original.

But otherwise, I'd have to say, controversially, Swallowtail Cut on Scything Blade.

Consume Will...

Haha *Chuckles in an evil fashion*

^ Totally needs nerfing.

My fav is the roomsweeper one that causes a stun after being hit. By far my most useful CC for pvp or pve.