Depends on who is fighting most recently and how dominant they have been.
However, the top 3 are Conor, Jones, and Izzy even when they are not fighting.
1) Izzy gets the absolute most amount of hate bar none. The crimes his haters accuse him of are touching his dog inappropriately, not being an "exciting" fighter, and being a weeb.
2) Jones: some of the hate can be attributed to the fact that he is the most accomplished active fighter in MMA and possibly of all time (he is GOAT imo), so naturally he is very visible and thus it will bring a ton of haters and admirers. Doesn't help that he has actually committed crimes.
3) Conor: was at the top of the world then spiralled out of control. The hate he gets was mostly from the Khabib fight which split people based on ideology. However, I think he soon lost even the people who hated Khabib with how he acted after that. The way he lashed out at Dustin and his wife, assaulting random people, and all the accusations against him. Also, much like Jones, the amount of hate can also be attributed to his success and visibility as the single most famous MMA fighter in history.