Americans know Joe Calzaghe as an undefeated British (OK, Welsh, but Americans have trouble with that distinction) champion who shocked the boxing world by upsetting Jeff Lacy in what many regard as Joe's first fight against a truly world class opponent. This account could use a little nuance.
For one, Joe regards himself as of mixed ancestry with his Welsh mother and Sardinian father/trainer. Few are aware that in addition to being called "The Pride of Wales," Calzaghe has a second nickname, "The Italian Dragon." Since he's lived in the U.K. his entire life, though, they can certainly claim him as their own.
Second, most have forgotten how Calzaghe won his super-middleweight title in the first place, which was against the legendary Chris Eubank. Some have argued that Eubank was a spent fighter at the time, but going into the fight Eubank had only lost to one opponent, Steven Collins (who he lost to twice), and he was only 31 years old. Eubank, who Calzaghe to this day claims was his toughest opponent, was never the same afterwards. In fact, Joe sent him into retirement, though he later returned in a comeback attempt that didn't go anywhere and lowered him in the eyes of many. If you look at Calzaghe's opponents since, you mostly see a lot of good records attached to names you've never heard of, which is why going into the Lacy fight, Joe was widely viewed as an overrated and heavily protected fighter. Even in the U.K. bookmakers favored Lacy, and Americans almost universally anticipated a Lacy blowout.
The result was one of the most impressive wins ever by a British boxer, an absolutely crushing defeat that left Lacy a mess and cast doubt on the future of his career. In that fight, Joe landed over 1000 punches (yes, a thousand), was barely hit, and in general made one of the world's most heralded boxers look like a rank amateur. Yet some still weren't impressed, and you could read Americans in various boxing forums finding excuses for Lacy and continuing to rip on Calzaghe. People who before the fight regarded Lacy as a kind of boxing Deity-in-the-making relegated him to the dustbin overnight, claiming he was always a phony and never that good in the first place.
Another storyline that emerged was that Joe was an asshole to Lacy and a prick in general, an opinion unsupported by Joe's actual comments after the fight, where Calzaghe described Lacy as a true champion who would make his comeback. When the Lacy camp asked for a rematch, Joe was less gracious, but let's be real here: if there was ever a rematch that didn't need to happen, that was it.
Regardless of your opinion of Calzaghe, here are some facts: as an amateur, Joe won three consecutive ABA titles, each at different weights (the only British boxer to do so); since then, as we all know, he has not lost a fight; he has defended his title an astonishing 20 times, and he is the longest reigning world champion in boxing. If he beats Mikkel Kessler, he will surpass the 20 defenses made by Bernard Hopkins and Larry Holmes, leaving him four short of Joe Louis's 25 defenses. He will also unify the WBO/WBA/WBC/Ring Magazine Super Middleweight titles and place himself five fights away from equaling Rocky Marciano's unbeaten record.
Somewhat under the radar, Calzaghe has positioned himself to become one of the greatest PFP fighters ever in the sport. To do so, he needs some more big wins, and if he can achieve the records cited above, that wouldn't hurt either. After Kessler, he will likely face either Bernard Hopkins or the winner of the Jermaine Taylor/Kelly Pavlik bout. If Calzaghe can beat Kessler, Hopkins, and Taylor before retiring (an enormous "if," I know), a strong case could be made for him as superior to Mayweather as a PFP champion. He would be remembered by many as the greatest British boxer ever to put on gloves.
I would summarize Calzaghe's strengths like this: first, he remains incredibly busy when at his best and what's more, incredibly accurate. This accuracy is all the more impressive when you look at the actual punches, many of which look like they shouldn't land, yet somehow do. He comes at you from everywhere. I would argue that no boxer alive has a greater variety of punches that can land. This versatility is complemented by his awkward southpaw stance. You can have fought a lot of good boxers and still not be ready for Joe. Second, while not exactly a knockout artist (though his 32 KOs isn't shabby), he is a power puncher, as Lacy's post-fight mug demonstrated. Third, Joe is a tough mo fo--he's only been knocked down twice, and he once fought to a 12 round UD, despite having broke his hand in the third. Finally, he is also a smart mo fo. You wouldn't call him slick, but his ring intelligence is obvious. He knows when to pressure and when to pivot, and despite his aggressiveness, he almost never makes mistakes.
I'm picking Calzaghe over Kessler in a fairly close and respectably entertaining fight. I would also choose Joe over either Taylor or Pavlik, based on what I have seen recently. Calzaghe/Hopkins would be hard to call. The Hopkins who fought Wright would give anyone tremendous problems, but Joe is both stronger and more active than Winky, which could give him the advantage if it went to the cards.