Mine's Super C! I've yet to see a game on current consoles that is as addictive and all around perfect as this game. The PS2 is supposed to be getting an updated version, and it'd better be better than those crappy PS1 versions.
Now, before someone jumps down my throat for the weird options, let me explain myself:
All three Super Mario Bros. are pretty much the same, despite Mario 2's penchant for Japanese weirdness.
Contra and Super C are grouped together because the “C” in Super C stands for Contra. Contra Force just sucked.
TMNT 2 and 3 are the same type of game. TMNT 1 is totally different in both style and genre.
Castlevania 1 and 3 are action games that can be played through in one sitting. Castlevania 2 is a loooooooonng adventure game.
Despite the fact that Zelda1 is top down and Zelda2 is a side scroller, they are the same type of game. Zelda2 is more advanced though.
Spy Hunter and Super Spy Hunter are the same type of game, although I think that Super Spy Hunter is the better of the two by far.
R.C. Pro Am because I had to throw in a “true” racing game (Spy Hunter doesn't cut it) and Tetris because I needed a puzzle game.