Here are some of my favorites
Chrono Trigger Dance Party (Afrika Bambaataa vs Chrono Trigger) - ASKAAK
http://funkyheroes.ytmnd.com/
Beastie Boys vs. FF6 (Body Movin' - Magic House Remix) - SirLemming
http://beastieboysff6.ytmnd.com/
Hova Turnabout Sisters (Jay Z vs Phoenix Wright) - Mohrdikai (WARNING NSFW!!!)
http://hovaturnaboutsisters.ytmnd.com/