Top HW boxers would be terrible, they are far too tall. It is well known that it is easier to takedown tall fighters vs shorter fighters. Also generally speaking, applying the MMaths rule of thumb. A pure kick boxer beats a pure boxer, a pure wrestler beats both and a pure bjj artist trumps all. Hence the reason...
bjj > wrestling > kickboxing > boxing
*But when combined, i believe a wrestler is best*
Also Mark Hunt was a terrible boxer but a good mma fighter and fought for the UFC HW title.
Badr Hari was the greatest kickboxer ever but a terrible mma fighter.
James Toney was a decorated HW boxer but a terrible mma fighter.