Oh, I remember Sephiroth. Yeah, that guy was tough. Been too long since I did it, can't remember how many tries it took me or how long.
I never really played Kingdom Hearts, but I remember Sephiroth being pretty difficult in FFVII. Of course, that was years ago.
Smithy from Super Mario RPG
Spoiler I never owned SMRPG as a kid, but I'd rent it from the video store on the weekends constantly. I'm sure we spent way more money renting it than we would have just buying the damn game, but that's what happened. It took me several weekends of battling Smithy before I managed to defeat him. SMRPG was also my first RPG of this style, so I wasn't very good at it and I didn't really understand how to grind or level efficiently, so it was a lot harder for me than it is now.
I detested Smithy. SMRPG was one of my favorite games as a kid and I thought it was fairly easy, but Smithy was a pain. I recall Johnny (the shark captain) giving me problems as a child, but I beat him pretty easily when I replayed the game about a year ago.
Eh, once you figure out what to do, he becomes quite easy. However, I had difficulty with that electric guy in MSG3 when there's a timer, especially when you're just trying to make him pass out. In addition, Ocelot in MSG4 was difficult for the first...10 tries. The Fury was more [censored] annoying than anything, and the Fear can be a [censored], unless you figure out a certain weakness of his and exploit it.
I had exactly 100 game overs (and countless alerts) when I played MGS3 on its most difficult setting. The Volgin fight contributed to my dying a lot.