Bagheera.
Much like Phoenix Wright in the Marvel vs. Capcom series, the Trix Rabbit is dependent upon his Trix powerups (in the original spherical shape) in order to be an effective fighter. Should the Trix Rabbit be able to obtain three bowls of Trix, he is on par with Akuma (if he gets six, it's Rare Akuma).
Unfortunately, Trix here is a joke character, and every time he is about to collect a powerup it immediately disappears, stolen by some obnoxious children who don't care in the slightest that he bought and paid for that cereal with HIS OWN MONEY and is unquestionably its owner and has the right to do with it as he wishes.
Bagheera, being a formidable fighter with his Enigmatic Wisdom and Po-Faced Slap specials, shreds the Trix Rabbit in seconds.
They can't all be upsets, folks.