Biggest one I can think of is the Sister Friede and Father Ariandel fight from Dark Souls 3's Ashes of Ariandel DLC. Up to that point, Dark Souls bosses throughout the series came in two flavours: either the fight is one single phase, or emptying a boss's health bar triggers a cutscene and a second phase starts. This is the way.
Sister Friede doesn't exactly play by the rules, though. During her first phase, you kill her, she gets back up with some help from her father Ariandel. They proceed to kick your ass together, O&S-style. So you beat them, again. You get a victory notification on-screen and the game drops you a big piece of valuable loot. You set the controller down to catch your breath, relieved that it's finally over, and while you're not paying close attention the stupid wench gets back up AGAIN and basically goes super saiyan on you for a third phase of the fight! Also, did I mention she tends to be invisible a lot while you're fighting her?
The whole thing is ridiculously hard and insanely epic. Beating this boss was more cathartic than actually finishing the campaign or the final boss of the other DLC. He was hard, but Friede is something else. You get to wield her sweet-ass scythe for your troubles and it's just as well that it's cool as heck because getting it is going to take years off your life.