Definitely OmniCC. Not knowing when my CDs are up again can literally kill me.
Pitbull as a close second, the standard frames wasting 2/3 of their size for useless clutter are horrible, and the new party/raid frames are way to small.
Everything else I can manage to live without.. temporarily
(Meaning I'll instantly install NeedToKnow, Dominos, OneBag, ChocolateBar+Plugins, RatingBuster and Recount as soon as Addons are enabled)