The best Mavel vs Capcom crossovers, Nintendo could have

Nintendo vs SEGA: Sonic Fighters, Virtual Fighter, Fighting Vipers
Nintendo vs Square-Enix:

• Link,vs Fighter 
• Mario Kart vs, Little Big Planet Kart,
• Front Mission vs Advance Wars, 
• Ganon vs Vincent, 
• NESS vs JENOVA,

Nintendo vs Apple: Mario vs Angry Birds
Nintendo vs Capcom: Mario vs Mega Man/Ryu, Luigi vs Ken, Wario vs Dan,
Nintendo vs Playstation: Nintendo vs Square-Enix as well as Sly Cooper, Sack Boy,
Nintendo vs Xbox:
You can play Nintendo

• (Ninja Gaiden) vs Ninja Gaiden (Xbox and beyond!) 
• GoldenEye vs COD, 
• Perfect Dark vs Halo,
• Kid Icarus vs HALO, 
• Mario vs Madden