Ikaruga is definitely one of the best. Got a copy for my Gamecube ages ago when it came out and it's sooooo good.
That said, if there's one shmup I would recommend over all the others, it would have to be
Viewpoint. I'm not gonna type up all the reasons why it's awesome again, so if you're interested in it, you can
read my full review of the game here. It's insanely difficult and as awesome as they come.