This thread has shocked me to my very core. Clearly either people suck at Final Fantasy, or just bought it because of hype and not because they actually like RPGs.
First of all, Yuffie is an amazing character, why do you think she was so hard to get? I know she wasn't that hard for most people but if you never looked at a guide she wasn't easy to acquire. She had some amazing weapons with great Materia slots, she was quick, her level ratio was smaller than most RPG characters, and her LvL 3 and 4 limit breaks just pwned for damage.
Secondly, Sephiroth does kick so much ass. Moreso than most characters. Let's be honest, he was the clone of a freak who wielded a sword and he was only beaten because he was taken 8-1 and all of his opponents were masters of their arts. Plus Sephiroth never used Materia, he was naturally powerful, with the force of the lifestream within his very blood. And he cannot catch fire, that's just useful.
And Vincent's limit breaks didn't bring him down (the third one did) but Chaos? Holy crap, what a beast. And the Leatherface limit break (which I believe was called 'Nightmare') was just novelty to the highest degree.
However, I do agree that Cloud is crap. The movie AC really ruined him as a character. He was a kind of psychotic lifestream infused swordsman with a sword bigger than him. In the movie he was an Emo without the fringe and piercings. He did have a f***ing sweet bike though.
My most useless character ever was Shadow (from one of the earlier games can't remember which, 4 maybe? or 6?) I personally didn't like his play style, and the fact that scrolls have one use and gone was annoying. Especially when none of them were really that effective.
My favorite character ever was Cecil from 4. As a Dark Knight he was cool and as a Paladin he was kick-ass.