The guys I think are debatable are Fred Warner, demario Davis, lavonte David, Devin white, Leonard, and Bobby Wagner (although he's probably past his prime and not quite that level anymore), and Micah parsons. I don't have time to look up stats right now. Roquan is elite in coverage, but his run stopping will be put to the test with mediocre DTs in front of him this year. Also his inability to cause fumbles with his immense tackle numbers is pretty shocking
Quan led the NFL in run stops per PFF with 67. Warner and Campbell had 53, Wagner 51, Warner 48, White 46, Davis 39 and Leonard 37. So Smith had 14 more than the next closest guy. Quan was 2nd last year with 66 with David being the next best at 57. So Quan has made more plays in the run ge than anyone else over the last 2 years.
As for coverage he was 2nd in passer rating in 2020 ahead of all those guys. This year he was 15th overall behind David, Parsons and Leonard.
So the only other guy I could entertain being better than him for sure is Parsons who also has way more sacks. But over the last 2 years Quan has had better coverage and run stopping stats than all those guys.
As for playing in a 4-3, there is no one extra DL to take on blocks and the WILL is naturally unblocked a lot in the Cover 2 hence why it is the most important LB position. Hence why the WILL is typically filled by undersized guys with speed who can chase down plays unblocked which is Quan's main strength.