Lesnar's career was pretty inflated based on how he knocked down Health Herring in this fight. You take away this highlight and Brock's career pretty much falls apart since a lot of his post Herring wins were a mix of perfect matchups and luck. Also consider Brock's first win in MMA was Kim Min-Soo, who finished his MMA career with a record of 3-7. I'm not saying his wins weren't legit, but lets look deeper at his and his opponent's records...
- After Heath Herring lost this UD to Lesnar, he would go on to lose to Amir Aliakbari and Satoshi Ishii and then retire. Prior to Lesnar, Herring had 8 MMA fights where he mixed wins and loses, going 4-4. He couldn't even put together a 2 fight win streak. You could argue Herring was a journeyman or washed sometime around 2004, 4 years before facing Lesnar.
- Brock legitimately beat Randy Couture, but he probably outweighed him by 60+ lbs and had the perfect stylistic matchup against him (another wrestler who didn't have true KO power and would clinch).
- Mir was probably Brock's best win. This was also a very favorable stylistic matchup for him.
- Carwin should have never lost to Brock. Most ref's would have stopped that fight. If Carwin picked his shots, he would have eventually gotten the stoppage. Carwin was just a better all around fighter than Brock.
- Brock was never beating Cain or Ubereem stylistically, even pre-disease.
- Mark Hunt was probably the best stylistic matchup Brock ever got. A pure striker with mediocre takedown defense.
You consider all that, JDS, Carwin (in a rematch), Cain, Ubereem, Ngannou, Stipe, Aspinall, Fedor, and a lot of recent HWs would probably beat Lesnar.