perfect cherry blossom, imperishable night and other touhou-games (i'm not able to configure the keys. has anybody done that?)
+1. You may not like the strange japanese aesthetic, but if you can get past that I think the gameplay is on par with any other game in this thread. Just an amazing series for shmup fans. Here is how I found out about it, this video explains the Touhou games as well as other bullet-hell shooters (danmaku):
http://www.blisteredthumbs.net/2011/02/ycpt-3-18-bullet-hell-games/More:
http://en.wikipedia.org/wiki/TouhouNewcomers should start with the eight game in the main Touhou series, Imperishable Night, which is arguably the best. The first five are not so hot. I don't know about keys, but joystick/gamepad buttons can be reconfigured.
Other suggestions:
GGPO - play fighting games from MAME online when no one's around for local multiplayer
Rondo of Blood - my favorite Castlevania game, runs on a PC Engine CD emulator
Number Munchers - play it again and tell me you're not having fun
Hydorah (mentioned above)
Tumiki Fighters