/ice because the blaster level AoEs and Sleet is a good thing. /dark for added -tohit mitigation to your shields and T.T. keeps things in Sleet. /sonic because it's just unfair what it does against hard targets.
What Amy said. Ice and Sonic are the big ones for area and single target damage respectively, while dark has synergy with cold.
I was going to say not Psychic Blast as I too have one of these at 50 but I see you already figured that one out.
I have a Cold/Sonic and AV's just melt insanely fast. Being able to debuff 100%-150% resistance is very overpowering. Your team will do plenty of damage. You shouldn't worry about how much damage you do. Also, defense debuffs are almost worthless. Tons of power sets (not just Defenders) have them, so chances are someone else on your team can debuff defenses. It is incredibly easy to get 95% chance to hit everything when you play on a team. When soloing, accuracy enhancements and set bonuses should be more than enough to hit even-level critters once you get to SO's.
Here is my build:
Code:
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If I didn't have a self-imposed rule of not repeating powers, I would've gone Cold/Son or Cold/Ice. As it is, I went Cold/Dark, and it works out pretty well. Sleet + TT for a soft immob while flopping (and no running out of Sleet), Shields + -to hit from the Dark blasts, etc. Mind you, I'd much rather Heat Loss 11+ mobs then Sleet & (blaster level) Blizzard w/o an end crash (Dark's nuke is pretty meh by comparison). 
An Offensive Guide to Ice Melee
I currently have a Cold/Psy...but I'm a little dissatisfied with the blast set. Any recommendations for a debuff monster?