I suppose he wouldn't make the list if we're factoring in longevity, but Alonzo Mourning might be the greatest defender who's routinely overlooked in these discussions. To give an idea of his impact: a 36yo post-kidney disease Mourning anchored a 98.6 DRTG that held opponents to 39.9% eFG when he was on the court during Miami's 2006 title run (those numbers jumped to 104.3 and 49.7% with Shaq on). Over half of Miami's playoff games came vs the 1st ranked offense Mavs (111.8 ORTG) and 4th ranked Pistons (110.8 ORTG). In the RS Miami posted a 100.3 DRTG allowing 44.7% eFG with Zo on vs 106.6 and 49.2% with Zo off (league avg 106.2 and 49.0%), to put it another way: the 2006 Heat's 9th ranked defense was a below average defense outside of Zo's 1302 mins of playing time
Prime Zo's DRTG impact (on, off, league avg):
1997 RS: 97.6, 103.4, 106.7
1997 PS: 96.0, 106.2, 107.4
1998 RS: 97.3, 103.6, 105.0
1998 PS: 102.0, 107.1, 105.6
1999 RS: 94.0, 105.2, 102.2
1999 PS: 100.9, 111.3, 101.8
2000 RS: 99.6, 100.0, 104.1
2000 PS: 94.7, 90.8, 104.2
Zo was likely only about 6'9" tall on a good day
But he was as strong as an ox with a massive 7'6" wingspan and possessed one of the most impressive combos of defensive motor + rim-protecting instincts I've ever seen. Zo was just entering his peak winning b2b DPOYs in '99 and '00 (during perhaps the most defensive-slanted era in NBA history) before his career was irreparably derailed by kidney disease at age 30 mere weeks after leading the gold winning 2000 men's Olympic basketball team in mpg
As a bonus: though Zo was somewhat undersized for his era, his mobility/footspeed/recovery/reflexes would project to translate very well to the current game. 90s Zo always brought far more value as a team defender than individual defender, and this would be further optimized today with the high premium placed on bigs capable of anchoring team defenses as mobile rim-protectors who can defend in space. Zo was one of these guys who had the unique ability to sometimes appear uncoordinated and clumsy on offense while simultaneously being super coordinated and agile on defense, able to cover broad swaths of real estate despite being bound by illegal defense rules in his prime (the elimination of which was in part how he was still able to deliver such defensive impact when returning to full-time play in 2006)
