Overall, the OP's post makes really good points but he likely went too far saying this was the best UM defense ever. The points per game given up were significantly lower in 2001 (and we played many highly ranked teams that year) but there is no question that this 2018 defense played at an extremely high level (especially in light of the very poor punting and our production on offense). Had we punted decently the PPG in 2018 would be probably be 3.0-4.0 PPG lower (around 15.2 but that's still higher than approx 10 in 2001). We gave up huge chunks of yardage due to shanks & poor punting which provided way too many short fields for our opponents and is really demoralizing for a defense.
As good as our current crop are, our LBs and DBs were better in 2001. Ed Reed is a Hall of Famer and was an incredible ball hawk. He was also a great runner in the open field. Buchanan was a phenomenal punt returner in addition to being an awesome DB. Rumph may not have had a great pro career but he was a first round pick. We had three starting DBs picked in the first round off that 2001 team. That record may never be beaten. Bandy, Jaquan and Redwine are very good though and deserve major props for their play.
Gerald Willis is better than Walters but about a push with William Joseph. Gerald Willis had a heck of a year though in 2018 and deserves major props for anchoring a great defense. If they work hard and continue to get stronger, Garvin & Rousseau may end up as better NFL prospects than our DEs in 2001 but they were both young this year. Based on production, I'd take the 2001 DEs over this year's group.
Manny Diaz and the starters deserve a ton of credit for a great performance this year.