Ensemble Darkhorse: Psylocke. Not only did she reappear in Marvel Super Heroes (alongside Wolverine, Magneto, and Juggernaut), but she remains a top choice in fan polls for Marvel vs. Capcom 3, consistently placing in the Top 5. Being one of the high-tier characters in MvC2 as well as a useful Helper in MvC1 most likely helped.
Suspiciously Similar Song: Psylocke's theme has some hint of the I Love Lucy theme, while Magneto's has some touch of Thriller.
Unexpected Character: Silver Samurai, particular since he made it into the game over better known X-Men villains like Sabretooth and Mystique. It's possible his concept appealed to the Japanese developers at Capcom.