Vergil, from Devil May Cry 3 and 5. One of the best boss fights in action game history, in both games, and he will make you work for your win. Do not expect to beat him on the first try, unless maybe you're on the easy difficulty. In DMC5 on Dante Must Die, the game's highest non-gimmicky difficulty, he's so powerful that you actually can't use your super mode (Sin Devil Trigger) to try and wrack up a ton of damage on him (a legitimate strategy on lower difficulties), because even though you can armor through his attacks in that mode, he will just kill you with a few moves if you try - because your armor in that form ignores hitstun, but not damage, and he does just that much damage. I am legitimately surprised and proud to have beaten him on that difficulty.

On a similar note, while she's an optional bonus boss rather than the final boss, I feel like I should mention Sigrun/the Valkyrie Queen from God of War '18 anyway. She has all of the abilities of the other Valkyrie and then some, and those were already not easy fights unless you over-leveled. Took me literal hours of retries to beat her (I believe on hard difficulty, though I'm not 100% sure).

Oh geez, thinking of other action games: Senator Armstrong from Metal Gear Rising: Revengeance. Even facing him the first time on normal difficulty it took a few tries to beat him. Facing him on Very Hard, the game's second-highest difficulty, was tough enough that I never actually played through the game on Revengeance (the hardest difficulty) - decided that beating Very Hard was accomplishment enough.

While (like in any RPG) it's dependent on how much you leveled, I recall the final boss of Persona 3 being particularly difficult the first time I faced it as well (not naming because I refuse to spoil a Persona game, even if it is 14 years old). That one took hours just because the fight can take that long, IIRC.