Both Halo 3 and SSMB are grossly over-rated. There's better shooters than Halo I can play on the PC, and SSMB is a bit of a laugh but boring after a couple of matches.

For me it's probably between Team Fortress 2, WoW, Guitar Hero, Diablo 2, and if you want some old school action Gunstar Heroes, which to this day remains the best two-player game ever.