My favourites, in no particular order:

Nigel Benn - absolute warrior, an animal ready to fight anyone
Jimmy Wilde - just the best flywieght who ever lived. Period
Joe Calzaghe - world class, legendary record, a winner
Randy Turpin - a lightheavy in a middleweight body. Beat the great Sugar Ray, then went nuts and killed himself ... classy.
Jem Mace - the original bareknuckle king of the ring.

Least (and I find this hard because I respect anyone who climbs into the ring generally)

Naseem Hamed and Scott Harrison - wasted talent and unpleasant people who never respected their opponents, their sport or, ultimately, themselves.