Diablo I: The Butcher
I can sort of understand why this bastard made an impression on young 'uns who first played the game back in '97, but as an adult, this boss is more a point of contention. In that "ah, fresh meat!" is memorable, but it's basically a jump scare, and you're practically guaranteed to die the first time because once you initiate the fight, there's little you can do to avoid it. So, how do you beat the Butcher? By dying, learning from your mistake, grinding, then do it again, stay in one place, and keep clicking. Part of why I like the D3 Butcher more in that there's far more strategy to it (i.e. you actually need to move).
So, yes, he is worth a mention of "hardest first boss," but for all the wrong reasons.