Prime Mike Tyson had great head movement, the way he slipped and countered with such power and precision was very impressive.
For full time proper HW's... Walcott, Byrd, Louis have the best ones - in terms of being the hardest to hit / get to? Holmes, Tyson and Wlad are up there too. At there bests all these guys were horrible match ups and very hard to get to - Old George was awesome too, the worst H2H here but you can learn a lot watching him.
Forgot Usyk, good defence, guy you'd be able to copy because it's so talentless and relies on stupid things like boxing - his tap, tap hands couldn't carry Buster Mathis's heavy 70s jock doe.
Prime Tyson, but when Lewis and Wlad especially fought safe, they where very difficult to tag I still think prime Mike had the best defense, considering he wasn't relying on height and reach. However, his short stature sometimes helped in the defensive department