I have a tie with 4 characters.
I've won every competition I've entered with one of these three: Pit, Falco, or King Dedede.
Pit because the arrows are cheap as hell and one of the best for interceptions.
Falco because of infinite gun combo and amazing interceptions plus ground combos.
King Dedede because he is the best control character EVER. Infinite grab combo, plus insane hitting power (albeit a bit slow), floating jumps plus a super high up b move, PLUS his weight makes him get hit less of a distance than other characters, PLUS he has an amazing range attack to compliment his close range beatdowns.
The last person is Diddy Kong. Amazing amount of ground combos, air combos, and the banana peels are amazingly op. You run at your opponent and throw one, it stuns him for like a quarter of a second in which case then you can activate a myriad of different combos. I've lost a couple competitions using him because I'm still perfecting his up b.