AAD is the big one. but mostly parties. people really seem to go there more to socialize. I would say that only 1/4 talks I've gone to was actually good enough for the "national" stage.
Summer AAD is much smaller
SID is pretty big, but is very research oriented. good quality talks
Dermpath, Peds, and Surgery also have their own meetings
Hawaii Derm is the best clinical conference i've been to. talks were consistently awesome.