Hellsinker.

I kept hearing it's a "one-of-a-kind shmup" and it's a "shmup that somehow plays like an action game", so I tried it twice and couldn't see why it's praised.

Third time I was like "OK, ima read the manual, read guides, and give it one final shot." and holy shit, what a game. I'll probably be playing it forever, TBH.

Up there with Crimzon Clover World EXplosion for me now.