If Genesis is an option, I pick him. Hell, can I remove the entire compilation instead?

If not, then I have to go with Sephiroth. I actually don't mind him as a character, but the plot could be several times more interesting without him. If anything, Jenova should have been the main villain. It doesn't help that the crappy writers completely ruined Sephiroth's entire character after FFVII. The moron literally doesn't even use a fraction of his power on Cloud, in AC. Why? Umm... because he's arrogant. Yeah... that's not ridiculous or anything... >_>

So yeah, there are characters in the game that I hate far more than Sephiroth. At the same time I feel that the plot could be several times more interesting without him though(and none of his ridiculous AC antics).

I suppose a case could be made for Tifa as well, since she hurts the plot at times too.