I can not believe I was the first to vote for Marvellous Marvin Hagler!! In a period when the best middleweights of all time were all on the scene together, Hagler was the cream of the crop, just. His fight against "Hitman" Hearns is one of the greatest of all time.
Calzaghe is number two on my list. As mentioned before, top of the tree for ten years - you can NOT argue with that.
Ali was incredible, but fought on too long and we have all seen the sad results. The guy will always be a legend, but surely a good boxer knows when to quit.
Only Masan's Saddle has mentioned Lennox Lewis. This is a guy that eventually (after a couple of cock-ups) defeated every single one of his peers and retired as undefeated heavyweight champion of the world, holding all the major belts at the time. Very few boxers have achieved this.
And another personal favourite is Barry McGuigan, and outstanding boxer and incredible character, who is still one of the most intelligent and well spoken advocates of "the Noble Art" you could ever meet.