Underworld was pretty sick! I mainly saw most of their set because I was trying to get in a good position in the crowd for Deadmau5, but ended up enjoying them a lot more than I would have expected. (and btw, I ended up being about 20 feet away from the cube in the 60k+ person crowd!)
Probably my favorite part of Ultra: The Carl Cox tent. That place was crazy & banging day and night.