No offense to Lewis, but boy HW is thin if he's the number one contender. Guy is always exhausted, has little to no ground game, and just doesn't strike me as a terribly skilled guy, nor a guy who trains very seriously. Some will say that's his back issue, but it is what it is. He's coming off the most boring fight in UFC history, and a fight where he got throttled for 14.5 minutes. That big right hand is devastating, but DC should be able dispose of him with ease.
I would give Lesnar a shot against Cormier at his absolute peak physical condition, like the Herring/Couture/Mir version. He's not in nearly the shape he was in back then, particularly muscularly. I wouldn't favor him even back then, but at least he'd have 50 quality pounds on DC. Today's version of Lesnar won't be able to take DC down, and we've all seen how his fights go when he doesn't have success in that area. The Lesnar that fought Cain was already past his physical prime due to the bouts with diverticulitis, and he took Cain down in about 15 seconds, so I have no doubt that version could've put DC on his back. I don't think today's version can.
I don't think Lewis is really the #1 contender, but I think he's got some things in his favor.
1) His resume is pretty good. He's 9-1 in his last 10 fights (only loss was to Hunt, 4 fights ago).
2) He's a fan favorite. Mainly because he's so funny.
3) I think DC thinks it's an easy fight. I don't think DC wants to risk the Brock fight against a very willing Stipe. Or Blades. Or Jones (if Jones was willing, which it sounds like he wasn't).
Peak, roided-out Brock vs. DC would be an interesting fight. I think DC still wins just because he's a much more well rounded fighter than Brock has ever been. How much does/did Brock cut to make 265? DC was around 250lb when he fought Stipe, so I'm not sure he'd even have that much of a weight advantage.
Edit: DC basically confirmed #3:
“I would not have accepted a rematch against StipeMiocic on short notice, he’s too good under these circumstances,” Cormier said. “I’m not saying Derrick isn’t good, but Stipe is capable of beating me in different ways. He does different things well. I think in this fight, Derrick has to knock me out to win.
”Usually, if a guy only has one path to victory, I do pretty well,” Cormier concluded. “Yes, I will admit the opponent factored into this.”