Arguably, Strong Sad. Sure, he's a fat, well-read elephant. Yeah, he is smarter than half the cast. But the only one who can beat him up is Strong Mad, and several times mentions that Strong Bad is no threat to him. On top of that, he has beaten Coach Z to an inch of his life, single-handedly stopped the invasion of King Bubgonzola, strategically beat down the resistance in Strong Badia the Free, is a high-ranking officer in the Homestarmy and was able to survive being run-down by a truck and having it parked on him with only short-term damage. Plus the ways he gets back at Strong Bad, including saving snowballs in the fridge since he was 8 just to pelt him with them, and in Baddest Of The Bands, tazing him repeatedly.
Pom Pom tops this. He is the Only Sane Man, and has made many a blockbuster. But don't tick him off, or he'll five-hit combo you into next week.