Heavyweight by far is the worst. It's almost always been the worst.
Stipe Miocic is a great champion and one of the greatest heavyweights ever...and with his skillset, he would struggle to crack the top 10 at lightweight.
Overeem and Arlovski are somehow still relevant a good 10+ years after their primes, and they had already been knocked out a bunch of times then. Now, both have noticeably slowed-down reactions and movements from all the head trauma...but that's still enough to be contenders. Hunt at 44 is a shadow of his former self, and his former self wasn't that good to begin with (being decimated by Mousasi when Hunt's grappling was only marginally worse than it is now) in terms of where MMA has evolved. Oleinik has beaten a ton of guys despite having awful stand-up, being slow as mud, 40 years old, having no more than decent wrestling, and one go-to submission.
Don't even get me started on Derrick "I used to train 30 minutes a day but I trained 2 whole hours daily for the N'Gannou fight!" Lewis. A woefully limited fighter who wouldn't even sniff the top 30 in a good division, let alone #2.
It's debatable which division is the best, but the worst is almost invariably going to be heavyweight.