There's too many awesome stages, I'm having a lot of trouble narrowing it down to three!
#1: Pictochat: Most of the music is pretty irritating but I love all the changes the stage undergoes, such a creative idea!
#2: Jungle Japes: I love the atmosphere and when the raging river sweeps aways my friends and they loathe me for picking the level it's too fun.
#3: Smashville: Great for keeping things simple. Plus, I love evrrrthing Animal Crossing, so the music just gets me nostalgic of the AC:GCN days and I love the little characters in the background.
Some honorable mentions include: Brinstar, Frigate Orpheon, Wario Ware Inc., Pokemon Stadium I, Shadow Moses Island, Corneria and Pirate Ship.