There are constant debates about "prime" fighters and who they would've/could've beat. In these debates usually a fighter is defined as in his prime when he was on a win streak and then classified as past his prime when the streak ends. This leaves out the possibility that his competition got better, or that the fighter stopped training as hard, or that the fighter made harmful lifestyle choices, quit using PEDs, or that his fighting style evolved. Even age isn't the defining factor since we see fighters like DC and Romero fighting at an elite level well into their late 30s. I think a prime fighter is one whose skillset, octagon experience, coaching, and overall fitness are all near the peak that the fighter ever reached. Let's say they are at least 8 out of 10 everywhere with 10 being the best they ever attained. By this definition many fan favorites were clearly in their prime when they started getting beat, while others like Bisping, didn't reach their prime until later in their career when some of these factors like coaching and skillset really peaked for them. Frankly I think many Sherdog favorites were absolutely in their prime when they started losing. They had tons of experience, were in great shape and had as good of coaching and skillset as they ever achieved -- and still they lost. Wins and Losses fighter-to-fighter mean more than any theory about "prime" this or that because in an actual fight all of these factors come into play.