I am working on a cold/sonic. Lets face it. buffing defense and buffing damage is leet.
Cold/Sonic hands down.
Screw end drain when you are stacking on that much -res. Especially given that you are teaming mostly
doing a Cold/Sonic right now with all the debuff of -res; -def; -regen etc from those two combos. People will be lining up drooling for help 
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welll altho i see the synergy of cold/sonic totally I find it very...err..passive. I went cold/ele...lol and i am not disappointed.
Basically with perma SC + heat loss..the mobs can't attack..at all....in teams this means I don't have to apply shields or even have a healer or even a tank, a troller is just needed to keep the mobs in a pack. Obviously end resistance mobs are gonna be a problem but hey, you can't have the whole cake can you lol?
I'm also working on a cold/elec. Its a decently fun build. Nice to sap the mobs of their endurance for when a few attacks get through your shields.
Going elec mastery for power sink, electric fence for a 3rd attack. Yes its DoT, but defender damage is so low, if you're attacking an enemy with ST blasts, the fact that one is a Dot, but also immobilizes is kinda a wash and is very nice for things like AV fights to keep them in place.
Plasmic's Guide to Sonic/Mental
Plasmic's Guide to Regeneration
Plasmic Fire - 50 Fire/Rad Victory Server
Well here's the deal, I took sonic for the -res but I have no intention of soloing this toon. My only interest is being useful to the team, in which case do you think that short circuit+heat loss is more useful than -res? Because in the first example I'm disabling the mobs from doing anything, but in the latter my team is doing more damage but the mobs are still active?
I am also thinking limitless INF builds here also, with the intention of throwing a whole lotta recharge at this toon or perhaps +def just for the fun of disabling entire spawns lol.