I'd wear the Halo suit over Donkey Kong's. Mario wouldn't stand a chance. (Gamer's Closet from 604republic, who look like they may have raided Glen Brogan's closet.)
Darth Vader playing hockey against Master Chief? Yeah, I could watch that game. Let me tell you, it's tough being Canadian and not caring about hockey. (Vader vs Masterchief by Tyler Edlin; Link via Geek Art)
A Spy vs. Spy - HALO mashup? That should absolutely be a playable mode! And just imagine the hilarious explosions. (Spartan VS Spartan by Kevin of Form & Fuction, available at RIPT apparel. I may have been too late posting this one. My bad.)