Im probably gonna get yelled at by die-hard FF fans. I have to say cloud, Vincent and sephiroth is in my top 10. For various reasons actually. First off, what makes Cloud annoying is how the Game portays him. He's supposed to be the tough, scarred, manly character. Too bad they gave him Super yellow, spiky hair and a purple jumpsuit. Way to give mixed impressions.
Sephy is annoying because of why he wanted to destroy the world. It was portrayed as a uber traumatic experience of seeing the cruelty of Shinra. In all reality, all he saw was something that was supposed to be his mother (spoiler alert!!!!!!) even though it really wasn't, locked up in a tube. That whole game was pretty annoying actually. I enjoyed the game, its just that there were so many parts to it that made no sense what-so-ever.
What annoyed me about Vincent is how he was portrayed after the game in Advent Children. How was Vincent portrayed in that weird movie? the wiseman who knew everything and the person you needed to turn to in the toughest of times. Hmm, that makes sense until you play the game again. I kid you not, vincent says no more than 50 words in the entire game and it is all in his own little parts. He says nothing in the main plot, he plays no major role, he's not even that good a character. Also, how can you be super wise to everything in the world when you've been asleep the past 20 years?