GBA baby.
Gunstar Super Heroes
Astro Boy
Sonic Advance series
Hajime No Ippo
Bit Generations
some faithful SNES ports
...the list goes on. Plus, during the GBA's reign it was pretty much the only place for high powered 2D experiences. As with every system library, not every game is a gem, but GBA holds a special place in my heart. SNES is great, but GBA is "better."
EDIT: Shit, forgot about Warioware too. Nintendo's most exciting franchise baby!