Aren't many of these games already listed just monochrome ports instead of new games only for Gameboy (Color)? Anyway, here is my list. Many have been repeated, but maybe there is something someone missed.
Metroid II
Megaman games (I think there were 5)
Ninja Turtles games (3)
Castlevania
Kirby's Dreamland games
Kirby's Pinball (must own)
Mario Land games (3)
Zelda Link's Awakening
Zelda Ages games
Pokemon (the originals, but you only need 1 since they are basically all the same)
Those are the only unique fun games I can think of off the top of my head. Obviously, there are others.