1. Capcom Vs. SNK 2: Mark of The Millennium 2001 - this game was just greatness. When they re-released the E.O. version for Xbox and Live, it was a done deal. I use to kill nikkas with Ryo, Sagat and Cammy.
2. X-Men Vs. Street Fighter - X! This is Xmen versus street fighter!!!!! Greatest of all time voice actor. shyt was just amazing. Playing it in the arcades back in the 90's was

The way the game looked, from the intros, screen select, loading screen and gameplay was so crisp and enjoyable. Imagine playing the supreme version on a Sega Saturn

hearing the voice actors of rouge, storm, and wolverine being played by the ones in the cartoon series was just
3. Marvel Vs. Capcom 3: Fate of Two Worlds - Only listed this because of nostalgia. It brought back memories of when me and my friends use to play MVC2. Playing against friends on PSN was so much fun. I only play against real people I know. I don't play any matches against people waiting in a lobby. Nope. It's not fair, they always spam the same moves and I lose. Besides the nostalgia factor, this game is in the bushes.
Honorable Mention:
SNK Vs. Capcom: SVC Chaos - everybody waited for this game to come out. To see the capcom characters designed in SNK's style was beautiful and worth the wait. Game was broken. The intros, dialogs and story kept me coming back for more. That's something that CVSSNK2 lacked. I don't care who hated on this game, playing this shyt in its prime on OG Xbox Live is something that I cherish.
Worst Crossover:
Street Fighter X/Cross Tekken: I hate this game. It sucks. Nothing more to be said.