If I had to pick one and only one, I'd go with Fearsome Stare. So subtle, yet so ridiculously powerful. It also led into the below.
Fearsome Stare + Tar Patch + Ion Core Final Judgment = My petite little dark/dark defender walks into a room full of bad guys, gives them the evil eye, Very Bad Things happen, and five seconds later she walks out of an empty room.
Also great fun:
Soul Storm + Petrifying Gaze - To render the boss who may have survived the above almost instantly helpless.
Executioner's Shot - The animation just perfectly fit the idea behind my mercenary Traps/DP Defender.
Confuse - Great in regular play, but find the right type of enemy and it just got ridiculous. Embalmed Vahz? Sorcerers? Force Field Generators? Bring em on!
Howling Twilight - I never got it to brew tea for me, but otherwise it pretty much did everything.
Pyronic Core Final Judgment - Do you really need a reason?
Assassin's Blade - See above.