Sephiroth from Kingdom Hearts 1 (NOT Final Mix, gtfo). I say him because I originally didn't care and didn't want to fight him. I knew he was hard and didn't see it as worth my time to go through the hassle of beating him, especially since I'm not a Final Fantasy fan and I couldn't give two cents about how much of a bad ass people keep making him out to be. Then one day a bunch of people in a live game stream chat of Kingdom Hearts 1:Final Mix said that he was easy. If I was drinking water at the time, I would have spit it out. I argued that he was hard and I could bring up direct experience and, if I wanted to, even character bios entries from web sites describing the boss fight showing he was hard as fuck. But of course my evidence was discounted and they just repeated that it was easy. At this point I was directly insulted by this and set out to try and beat him. His one hit kill move (even with the Second Chance attribute activated) couldn't save me and I died CONSTANTLY. I was even reporting what I was doing and how many times I died to this one guy in the chat who also said it was easy and he wasn't saying much in the way of strategy, the most he said from what I remember was to spam this one weird move name. Getting nervous and jittery at the start of the fight with that theme music and Sora having the keyblade finesse of a drunk cab driver with a club wasn't helping me get any closer either.
I really wanted to drop it and get on with my life after what seemed like my 30th failed attempt, but I couldn't. The insult of Sephiroth being "EASY" had drilled itself into my brain and I couldn't get it out. One I think the third or second night since I started trying, I was in bed and I had a thought of a strategy (on top of the other strategies I had come up with) I could use to beat him and stop him from using his one hit kill move on me, or if he was able to cast it, keep him away from me long enough so I could heal. It took me 3 more tries, but at lvl 82, after so much obvious utter bullshit from the people in chat lodged in my head I finally beat the fucker! Watching him float up into the air and dissipate in a magical puff cloud of destruction was the greatest feeling of relief I've ever felt in a game. It really felt like this MASSIVE crippling weight had been lifted from my shoulders. I had beaten Sephiroth, and by beating him and having that experience myself I know for 100% certainty that he was not easy, that he is a hard mother fucker, and anyone who says he's easy is a jackass who I'll shun and ignore any and all opinions they have about any game for all eternity.
I went back into the same chat room and said I beat him and that he's hard, but they said the same exact thing. That coupled with other reasons lead me to leave that chat room later and I've never been back.
I'm also NEVER doing that fight ever again as long as I can help it. I don't ever want to have that feeling of having an unbearable weight on my head. I felt like I could piss myself if I heard that theme song again.
(BTW, the person who was streaming the game said the same thing, and he couldn't beat the ice titan at level 50 or 60 when that was the original level he said he could beat Sephiroth at. When he did beat Sephiroth, he was 1 level over the level I beat him at, 83. Biggest fucking lier I've ever met in my life. I was originally level 72 or something when I started and looking at it, if i knew the strategy I came up with then, I possibly could have beaten him at that lower level.)