I love GBC games! X3
My favorites are:
Pokémon Gold, Silver, Crytsal, Yellow (USA), Ice, and Brown
It's Pokémon, nuff said, sweetie.
The Legend of Zelda: Oracle of Ages and Seasons, and also Link's Awakening DX.
Zelda on the go, what more could you want? I actually prefer Ages out of the three for it was different, harder, and it was blue, had better puzzles in my personal opinion.
Kirby Tilt 'n Tumble
I loved this Kirby game for it's unique controls and gameplay and the fact it was just fun for me. ^^
My number one favorite though might just be Super Mario Bros DX. It was basically portable SMB1 + 2, with a lot of extras. I'd admit though, those extras kinda gave me a reason to play through every level instead of skipping to World 8 like I normally do on my NES. XD