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.