#4 Mark Hunt
Akin to Robbie Lawler, Mark Hunt's biggest weakness has been his grappling. Hunt almost always ends up getting out-wrestled, and although his BJJ has improved in recent years, he still has a very hard time against good wrestlers -- with most of his fights ending as decision losses to the superior wrestlers.
Hunt fought out his contract with the UFC in 2018, and is now a free agent. Ergo, it's highly possible that 'Hunto' could end up fighting for either Bellator MMA or Rizin FF this year. Nevertheless, given that ONE Championship does have a Heavyweight division, Hunt could probably make a splash over there.
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The current ONE Heavyweight Champion is former UFC fighter Brandon Vera, who happens to be a great stylistic matchup for Hunt. The latter could potentially sign with ONE Championship, fight Vera, and win the ONE Heavyweight Title.
After all, Hunt is known for his KO power, and no matter how old a fighter gets, power is always the last thing to go.