The first thing I suggest is Capcom vs SNK 2: sure, you will find on GC/Xbox/PS2 too but, trust me, having on DC is really "special".
I know some European and USA cool games but the Japanese line up is totally unknown for me. Advices are absolutely welcome!
I have just bought from Amazon Metropolis Street Racing, it will arrive in few days!
If you like shmups (and have a good budget):
- Border Down
- Ikaruga (*)
- Trigger Heart Hexelica (*)
- Zero Gunner 2 (*)
- Mars Matrix
- Psyvariar 2 (also on Xbox OG)
Check Frame Gride too, it's a sort of mecha-action game in a fantasy world, made by From Software.
If you understand japanese, I reccomend Sakura Taisen games, good RPGs with nice stories.