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Only 6 have any chance: Mark, Dan, Fry, Bojack, Hank, and Xavier.
Bojack easily has the most raw size advantage, but he's old and sad. The moment somebody says something that reminds him of how empty his life is he'll start crying on the floor and neutralize himself. Not to mention he'd probably be outsmarted anyway.
Fry is incredibly lucky to have escaped with his life in a ton of different circumstances. He's also immune to anything that targets his brain because of time-travel pussy, not that that helps him here. If he has his 7-Leaf Clover he'd probably end up winning, or at least surviving, without even doing anything, but I assume we're talking base here.
Hank is passionate, and is less likely to start crying in the middle of a fight than Bojack or Fry. That said, he's not a horse like Bojack and is also middle-aged. The best he can do is do a football tackle or use a gardening tool as a weapon (which he'd be hesitant to since it's not its intended purpose).
Dan is vengeful, obsessed, and cunning, but without knowledge of who he's facing, or the ability to set up some elaborate scheme, he's not really ready to handle most of his enemies. If Elise can back him up he'd be in a much better position.
Xavier isn't actually a fighter, but the way he talks is near-nonsense, his appearance is incredibly jarring, and he often displays an ability to completely distort and fuck with reality to do some mundane task, which generally ends up only being done in a really roundabout nonsense manner.
Mark's honestly the dark horse. He deals with so much shit as a therapist in a monster-ridden New York City that he wouldn't be at all phased by the type of shit Fry's dealt with, Bojack's appearance, or Xavier in general. Mark's literally been to Hell on business, among other things. Plus, out of anybody, Mark has the most to gain from having a second. Callie would murder everybody else in a matter of moments to protect her boy toy.
Mark wins.