There's a gotcha in the caveat, 'how they treat their fans'. Evil Hat's fans are I'm sure quite on board with their ecstatic virtue signalling their hate for the man whose work they pilfered to sell their game, so technically they aren't treating their fans poorly. I think they're shitty people who made a garbage game system, but I'm not their fan, and they probably hope I die in a fire. Games Workshop really deserves the crown for abusing their fans relentlessly, I don't think any other tabletop gaming company comes close, though like Yabba pointed out they aren't directly making any RPG at the moment. Wizards? Ehhhhhhhhh. Maybe? Like Evil Hat I think they've been churning out garbage for years now, but I'm not sure that in and of itself counts as abusive to the fans. The whole OGL debacle indirectly abuses fans by curtailing the creators in the market that might make good compatible things, but also isn't direct fan abuse. Palladium in the past and TSR would both be strong contenders since they'd go out of their way to threaten fan content with lawyers, though I haven't heard of Palladium doing it recently and TSR is gone, so.
That indian guy who made the Wakanda for indians RPG and then demanded that white people buy copies to support him while also decrying them playing it, I think I'd probably vote for him. Dude was actively talking down to his fanbase of white saviors buying his shit entirely because it was indian, so I think he counts. Oh, also David Hill, since I think he ran a bunch of kickstarters and ran with the money.