There's a rare gem that I actually own for GameBoy color and that would be Super Mario Bros.Deluxe or DX. It has the old school original (although slightly modified- you can go back-screen a bit and look up or down) along with a very awesome challenge mode that'll keep you busy forever...and to top things off, it has a two player level race-to-the-flag mode, which you can also try beating Boo on your own...and a secret game, which is actually Mario Bros. 2 in Japan..."The Lost Levels" in the U.S.
RIN play's it flawlessly, too. But yeah, My favorites are Super Mario World 2:Yoshi's Island & any Mario Kart game.