It's a preference thing.
You'll hear a lot of Hollow Da Don as an answer, but that's one I just don't see. He lost (clear 2-1s) to Tsu Surf and Loaded Lux, and then he beat Joe Budden (a mainstream rapper who really didn't know what the fukk he was doing out there) in a very unspectacular fashion... but somehow he's the top of the hill. That's what he's done his last three battles.

Not taking anything away from him, he's a beast, but I just don't think he's the best, but I do respect someone's opinion who says he IS the best.
Charlie Clips would probably win a poll vote in here. Mook, Lux, Verb, Rex (before that dismantling by Clips) all can lay some kind of claim depending on who you talk to, I guess. Personally I rock with Surf and Daylyt the most, even though I wouldn't regard either as the best.