Hmm I can't remember them all, there's been so much. But there is one fight I remember that busted my balls hardcore. It was in Baldur's Gate 2, and there were many fights in that game which busted my balls, but there was this one big dragon... It took me literally about 15 attempts to finally win. I'm not sure if I was just being a spaz or if maybe I attempted it too early in the game or something, but it was really hard. But I was super determined.
In the end I only won because I did *everything* I possibly could to win. So my rogue I spent like 5 minutes before starting the fight just running around and placing every trap I could place. And then I cast every single buff all my characters could cast, and I popped any potions or wands or whatever else I had that might help. And then when the fight started, I would pause it literally every 1 second, and totally micro manage every movement. I debuffed the dragon with everything I had and then unloaded everything I had on it. Hell of a fight. I couldn't believe it when I finally won, it felt so good, and the mad loots made it even sweeter.
But that was the 90's. You just don't get stuff even close to that anymore. I played all the Mario and Sonic games as a kid, and they were really hard too. Gaming back then was amazing.
p.s. There is also one fight I have never won... There was a game called Best of the Best (also known as Panza Kick Boxing) on PC/SNES/Amiga. I could never beat Andre Panza. He was just such a badass and even when I got all my skills up to his level by beating all the other fighters, that last guy just always won. Frustrating as hell. Best fighting game ever though.