I could list a number of bosses from Touhou, but that's because it's freaking bullet hell.
One Touhou boss that does stand out however is Suika in IaMP, the first fighting spin off, who breaks the 4th wall with her last phase in a way that would impress Deadpool and puts pressure on you. First of all she breaks your stamina bar so you don't know how much you can guard before it breaks. For the record, this phase features HEAVY projectile spam and you need to use both the standing guard and the dash-guard (called grazing) if you want a chance at winning. The other bar she breaks is the health bar meaning the only hint that's she about to drop is a black hole that appears at the end which sucks you in. This is on top of her other phases, one of which is (insert nonsense word here) hard to damage unless your character is an ace with ranged attacks.