Classic: Super Mario Bros. 2 (USA)
Handheld: Super Mario Land 2
NSMB: Undecided (Do not own NSMB2 yet, NSMBW was a better game than NSMB, but NSMB had better modes, Luigi in single player, and the blue shell...)
3D Mario: Super Mario Sunshine and Super Mario Galaxy 2
Kart: Double Dash!! or DS. Haven't played MK7 yet.
Party: Undecided (only played MP2 and MP6. I like both)
Paper: The Thousand Year Door
And I haven't played any spin-offs other than Mario vs. DK DSiWare and Mario & Sonic Winter Olympics. Didn't play any of the sports games (except Tennis 64. Also played Golf 64 and didn't really like it), pinball land or any of the other RPG games yet.