Martian Manhunter
Note that if you include him, somebody will inevitably bring up "oh, well Martian Manhunter mind controls everybody and makes them gang up on Vegita, who's the only person in Z to demonstrate the ability to resist mind control."
Personally, I don't want to have to go rewatch all of Justice League just to find the one or two instances in the series that demonstrate he's not Godmode Sue like he is in the comics.
Decided to look up his abilities. He could curb-stomp every single one of the Z-fighters. His strength, invulnerability, and flight are equal to Superman, in his ordinary form, for the former two. The former two of those are able to be enhanced by him making his body denser. Speed is apparently equal to the Flash's. Stamina is nigh-inexhaustile and allows him to survive for long periods of time without food, water, or air naturally. Regenerative abilities are on par with Buu's, but even faster. He is able to incorporate outside matter into his form and shape-shift into the appearance of another Z-fighter. His is able to reduce his density to the point wherein he is intangible. He is able to appear invisible, even to beings that can see the entire electromagnetic spectrum. Extent of telepathic abilities are as follows:
He is also capable of reading the minds of all inhabitants of earth at once. His telepathic abilities also allow him to create realistic illusions; telepathically trace and locate people; shut down people's minds; brain blast; mental shield; influence thoughts; mind control people; manipulate memory; astral projection; possesion; induce sleep; reprogram or reorder minds; and transfer information directly into people's brains.
This is besides being able to launch Psionic Blasts and project a shield to augment his durability even further. He has nine senses in comparison to the human five, all of which sped up. He has a heat vision equivalent called 'Martian Vision', which is apparently far stronger, as it could harm Superman. He can manipulate cells, atoms, and subatomic particles perfectly.
Oh, and at one point he could rewrite the entirety of existence into something more conforming with what he desired. However that was a once only thing and he already used his up.
His weakness? He doesn't like being around fire. That is only true right now due to New 52 reverting him having gotten over this. Note that the fire does not reduce his powers any more, it just reminds him of how the rest of his race died.
Holy f***. How can you defeat this guy?