F-Zero! GX remains my favorite racing game ever. The worse thing, is that the series has a large cast and a setting which lends itself for a few spin-offs beyond racing! A fighting game in the style of Urban Reign? A beat 'em up like Double Dragon? An action bounty-hunting game?
SO MUCH POTENTIAL.
Golden Sun and Advance Wars, just because RPGs.
Also Wave Race and 1080 Snowboarding. Both GameCube entries were very good and fun to play.