Well, 99% of the time. I played as Jigglypuff a few times... Mario once or twice... maybe Fox... did Captain Falcon once. But only because it was an event in which it made you play as him forcibly for no reason.
Recently, though, I've done...
Roy vs. Mewtwo on Final Destination.
Dr. Mario vs. Young Link vs. Pichu on Big Blue.
Luigi vs. Gannondorf on Fountain of Dreams. (Though, I pressed the "random" button.)
Roy in Adventure Mode! (I don't remember if I even got to the Giga Bowser part at the end. I remember getting to the Bowser battle. But I don't recall if I got to fight Giga Bowser.)