Dragon Warrior Monsters. I got it along with a spare GBC charger and a Pikachu GBC carrying case thing. Many weekends were spent playing through that game...except the cartridge didn't save, so I would always lose my progress. I didn't care, though- the game was fun and was one I didn't mind restarting multiple times, even if I never got to the end. I've since found a working cartridge, but yeah.
Others include Super Mario World, LoZ: A Link to the Past, Mega Man Legends, Final Fantasy VII, and Jak and Daxter: The Precursor Legacy.