Gen from Super/Street Fighter IV
Few experiences in gaming are more satisfying than crushing opponents online with this tiny, geriatric, Leukemia-ridden Chinese man. He is, in terms of damage output and health, the weakest character in the game. The fact that he fights with two different stances, Crane and Mantis, makes his fighting style incredibly complex and technical, leading most players to abandon using him due to the difficulty involved.
But if you take the time to learn, you'll discover that Gen's tools are some of the very best SFIV has to offer. His jump has one of the lowest arcs (only Hakan's is lower), allowing you to easily set up one of the nastiest crossups in the game. He's also one of the only characters in the game who can reliably link from his full Super Combo to his full Ultra Combo.
If you can master Gen, you'll be unbeatable. Well, mostly. You still lose about half your lifebar from two hits. But still.