I haven't played much Super Gameboy since it does weird things with my TV, but last year sometime I got back into Gameboy stuff and the games have surprisingly held up very well. Most of these have been mentioned, but my favorites:
Donkey Kong 94 (My favorite Gameboy game, best played on a SGB.)
Super Mario Land (surprisingly still fun)
Super Mario Land 2 (weak for a Mario title, but still fun)
Mario Land 3: Wario Land (LOVE THIS GAME)
Castlevania (dat MUSIC, although C2 is better)
Kirby's Dreamland (easy, but fun)
Dr. Mario
TMNT: Fall of the Foot Clan