Depends on what you like. the Super Mario Land series is pretty good. So is Ducktales 2, and Castlevania Legends. I think that was the name of it. There were 3 Castlevania games or something like that for the Gameboy. It was the one that was "Super Game Boy enhanced", if I remember right.
I remember the first Super Mario Land being pretty good, as well as Link's Awakening. However, for every one of those, you got crap like Home Alone and T2: The Arcade Game.