I can't give you a long list of PS2 games since I only played them at a friends house, but I can recommend one that stood out:
The Red Star
It's both a Bullet Hell and a beat-em-up, has a performance-based upgrade system, and even 2-player co-op. Lots of fun, gets super difficult after the half-way point, and really unique.
The Red Star
It's both a Bullet Hell and a beat-em-up, has a performance-based upgrade system, and even 2-player co-op. Lots of fun, gets super difficult after the half-way point, and really unique.