The HW division isn't like the other divisions in MMA, where clear progress has been made, the sport and the fighters have evolved etc. The HW division is no different in terms of development than it was in 2003. Just look at the top 10 guys, look at the top 10 guys from 4 years ago. Werdum was just the champion, and he was one of the top guys 10 years ago.
When you talk about the best guy at HW ever, there is really only one correct answer currently, and that is Fedor. Fedor from the early 2000's to the late 2000's beats anyone. Why? He was on a different level athletically speaking. If you want to get technical, Fedor if he came from a weight cutting culture, would be a small 205'r. In fact, I think a case could be made that with diet to lean out his chubby frame, combined with water cutting, the guy could have gotten within spitting distance of 185. He obliterated all of the top guys from his era that he had the opportunity to fight.
Fedor has declined sharply, but that's no different than any other guy whose talent is predicated largely on speed, agility and quickness. Once that goes with age, you end up like Roy Jones Junior, arguably one of the greatest of all times, who got smashed on the regular at the end.
The evidence that the division hasn't progressed is just looking at the guys who have been in and around the top 10. Arlovski. Hunt. Werdum. Reem. BigNog held the title when he first came over. Sure, Cain came along and he was pretty good. Ask yourself, who did Cain beat though to assert himself? Guys who Fedor had already beaten, who Fedor beat when they were in their physical prime, not older men in decline like when Cain did it. In fact, BigNog from 2003-2004 is probably better than any HW currently in the game, a level of boxing that is higher than anyone else at HW I am aware of, plus world class JJ. The guy was a beast, and Fedor RAG DOLLED him.
The HW division in MMA is really a throw back division, where rounded guys are rare. Guys still tend to be specialists, because at HW you can get away with it so much better. You take prime Stipe, and prime Fedor, and I don't see any possible way Stipe wins. Stipe is tough, but so was/is Fedor. Is there a more relentless guy who doesn't know how to stop in all of MMA than Fedor? Maybe Rory McDonald. Stipe is way slower, way less dynamic, way more limited.
Yes Fedor fought some cans, but he also fought the absolute top guys in the sport who were legit as feck, and destroyed all of them. For almost a DECADE.