Dancing Mad is great, but I love Blackjack's theme, too. Heck, I love most the music in this game. Like, whatever the music in the Esper cave is called. If I knew it's name, I'd vote for that one, but I don't, so I'll vote for Blackjack's music (WoB Airship, rawr).

Edit: Went to vote and saw "The Opera" listed. If it's referring to the music played therein, I'll go with that, 'cause I really like singing that vocal theme Celes-posing-as-Maria does. xD