Almighty forces of the internet, I call upon thee to return this thread from beyond the grave!
Been a while since I played, but I my votes on Sonic. Not because of his move set, mind you, but because of his final smash. Not only is it insanely powerful, but Sonic can get the smash ball so easily that he is almost guaranteed to use at least twice per match.
See, while I agree that certain characters have simple and powerful tricks that can be exploited, the fact is that it's impossible to get a truly objective read on every character. You're never going to find two people who have the exact same skill level, the exact same play style, and the exact same amount of experience.
This is why I dislike Sonic; ha has one extremely reliable win condition, and nothing else stands out about him. Sure, he's fast and by no means a bad character, but he's not particularly effective or unique. But his final smash is such a powerful move, that he can pretty much just stall until the smash ball shows up, and use it to obliterate everyone else on stage.
I dislike Sonic because he makes the game into a frustrating cat and mouse exorcise; his cheap final smash is all that stands out about him.
If the smash ball isn't involved, I can't really say. Meta Knight and Pit are both very powerful, but I've beaten both of them with Ike and Link pretty frequently. The stage is also a factor; more agile characters with lots of melee attacks or more reliable recovery are obviously going to be more powerful on smaller stages like Final Destination, and if you have more reach or better ranged attacks, larger stages like Hyrule Temple will give you an advantage.
