Chael just made a video about this, and I thought I would chip in my two cents, offering four names that often get overlooked.
1)
Josh Thomson: Victim of bad decisions, and awful luck. Was on the verge of beating Yves before eating a flying head kick, and was arguably robbed against Bendo. Had top tier wins against JZ, Melendez, Noons, Ludwig, Nate, Franca, Healy. Excellent wrestler, very durable, great stamina, respectable striking, and could have easily been champion. In fact he should have been.
2)
Joseph Benavidez - Another case of a championship level fighter who never quite got the gold, but whose record and abilities speak for themselves. He is extremely well rounded, is dynamic, durable, tough as almighty Hell, and could have very easily been champion had MM not been around. His record is very impressive when you look at it closely. Great jitz, kicks, wrestling, stamina, power... the works.
3)
Jake Shields - Sort of an obvious pick. But it is amazing how much Shields is underrated these days. He was the Strikeforce
Middleweight champion, was on a 15 fight winning streak when he fought St Pierre for the title, with wins over the likes of: Kampmann, Okami, Dan Henderson, Pyle, Charuto, Mayhem, Daley, Lawler, Condit, and would later amass wins against Maia, Woodley and Akiyama. He also beat Sakurai earlier on. One of the best grapplers in the sport.
4)
Dennis Bermudez - The most controversial entry, for sure. I think the sport is not only about ability, but really there is a huge component of luck. Dennis was the whole package: extremely well rounded, great explosive wrestling, beautiful kicks, great cardio, great chin, and a total warrior. Stephens and others mentioned he ruined everyone in the gym... but he was victim to a bunch of bad decisions in a row (Elkins, Fili, Glenn...) and never quite got to show how good he was against the top of the lot. Yet he beat a (admittedly green) Holloway and in my estimation had he skills to compete with anyone in the sport. Hugely underrated.
Honorable mentions go to Grey Maynard and Tyson Griffin.