do you guys remember the boss in Diablo II, Duriel? Man that guy was overpowered. This all depended on what class you took, how you developed your skills trees, but overall in all my playthroughs I found Duriel always the hardest boss.
I remember he was hardest for my Barbarian, easiest for my Necro. My Barbarian was invincible before Duriel and after Duriel, but he was eaten many times during Duriel
Mephisto and Diablo himself was a pushover for me compared to him.