Most bosses in Elden Ring. I personally found Elden Ring to be incredibly disappointing, and the bosses were a big reason why. There's so many copy and paste bosses, most of whom aren't made anymore difficult in subsequent fights unless they are made into a two vs one battle, which is just cheap difficulty. The rest of the bosses, while having good design, are so overly aggressive, have unreasonable damage output, and input read so much that fighting them solo and in a more traditional Souls-like way is infuriating. The bosses were clearly design with summoning in mind, be it other players or the ashes. It kills the single player experience for me. The worst of the worst are Melania, Margit, any of the tree sentinels, and of course any multi-boss fight. I've heard the DLC only goes further with this flawed design philosophy.
Sekiro is my personal favourite FromSoft game, despite some of its flaws, and I like most of the bosses in the game. Demon of Hatred is trash though. It is just sooooooooooo tedious.