The Milwaukee Bucks are the best defensive unit in the NBA for plenty of reasons.
They have the perfect combination of personnel to make it tough for all sorts of opponents. Defense starts at the point of attack, and the Bucks have plenty of point-of-attack hounds that can put an enormous amount of ball pressure to bother ball handlers, navigate screens to stay attached around picks, and the ability to lock and trail to close airspace quickly.
Having the kind of on-ball personnel they have — Jrue Holiday, arguably the premier point-of-attack defender in the NBA, and others such as Jevon Carter, George Hill, and Wes Matthews — has empowered the Bucks to deploy the deep drop as their base pick-and-roll coverage.