Gogol Bordello and Chromeo both had huge dancing crowds for afternoon sets. I have a feeling Pretty Lights might make it in there if there are as many glowsticks flying through the air as I expect. LCD is #1 for sure though.
Non main stage acts that had quite the enthusiastic crowd and stand out in my mind as great sets: Ghostland Observatory late afternoon in 2007 was unexpectedly rad. The National on the Yeti in 08. The, to my knowledge, one and only performance by Crudo (Mike Patton project with Dan the Automator and a beatboxing asian chick, I think she was asian.) also in 2008. Deadmau5 in the dance tent 2009. Why? and Dirty Projectors in 2010.