What makes Rockhold better than Bisping and Jan at least is, in my view, that he established himself as the number one contender and then blasted out a reigning champion who was undefeated and had won all four title fights leading up to that fight.
Jan did not even defeat a LHW champion to earn the belt. He beat Reyes, who arguably lost to Oezdemir, and whose only real claim to fame is doing much better than expected against Jones. He also lost to Thiago Santos recently. Great, great late career resurgence that should be celebrated. But he’s never proven himself to be the best LHW on the planet.
Bisping is a little different. He beat the undisputed champion at the time for the belt. However, he never established himself as the number one contender, let alone the best middleweight. He stepped in to the title fight on a couple weeks notice because Weidman got injured and the other top middleweights were unavailable. He won fair and square against a guy who embarrassed him a few fights back and almost certainly took him lightly, then arguably lost to a very past-it Henderson, got stopped by a old, blown up GSP, and then got sparked by Kelvin. Just like Jan, Michael deserves great credit for persevering and getting to the title. But it’s quite possible that he was barely a top ten middleweight when he held the belt. Even if we ignore Rockhold: Weidman, Whittaker, Romero, Mousasi and Jacare would have been significant favorites against him, and guys like Machida, Brunson, Vitor, Hall and Gastelum are probably favored too. To put it in context, the guys ranked 11-13 when he was champ were Leites, Henderson and Kennedy — two guys he was in razor close decisions with and one guy who dominated him.
Forrest performed great against a legit champ in Rampage. It was kind of out of nowhere, and he was otherwise a back end ranked light heavyweight. Tough to evaluate. Earned the shot and the win, but IF he was ever the best LHW in the world it was for about a month.