Shikigami no Shiro 2 is a great choice, with so many characters (w/ 2 modes each) you can play that game for a loooooooong time.
I was hoping I could pop in and suggest some DC games that were cheap, but holy crap the prices have exploded since I last checked ($100 for Zero Gunner 2, wtf). The only games I can think of that are fairly cheap and don't have PS2 ports are Bangai-O and Mars Matrix.
Bangai-O isn't really a shmup in the normal sense, but it's a ton of fun, and Mars Matrix is a really good port with some ok extras.