I'd probably go Hagler, Pac, Whitaker, Camacho, Calzaghe. But that is only because I have not seen some of the earlier guys. Whitaker, from a skills perspective is probably #2 but I found his fights boring. Of all time I think you can only include Hagler, Whitaker and Pac, in that discussion. Part of the inclusion of Pac is his international appeal.
You know why everyone is saying Hagler, don't you? Because he was skilled AND exciting! That's what separates him from PW. He is a fan favorite. Who can forget where they were when Hearns-Hagler went toe to toe? Now, let's see, yeah, Pea was very fast and quick and moved well and hmm...who did he fight? lol I even remember where I was when I listened on the radio to the Hagler-Leonard fight. Hagler was exciting!
The holy trinity is Whitaker, Hagler and Pacquiao. You have Pac, a weight jumping freak. There's some criticism among Pacquiao's opposition at the higher weights, and them carefully picked. While they were calculated risks, could you imagine in 2005-07, when he had his trilogy with Morales, and the 2nd fight with Barrera, if you had said to people "In the next couple years, Pacquiao will destroy Ricky Hatton and Antonio Margarito, beat the hell out of Miguel Cotto, shutout Joshua Clottey, and be the #1 threat to Floyd mayweather"?? People would have said that is nuts. Pacquiao is extremely fast of hand and foot. I think his skills and smarts gets overlooked at times, perhaps because he is so unorthodox. His combos are unusual, but they work, and they aren't usually bull**** arm flurries without accuracy. He throws them at unusual angles to find the gaps in an opponent's defense. The whole point of combos is to create and exploit openings, and he does that extremely well. You have Hagler, a long-reigning, ATG of a traditional division (in a pretty solid era). Marvin was extremely well-rounded. technically sound with an iron chin. By all accounts, the Monroe loss was fair and Marvin was soundly beaten. But for his other losses and draws? Hagler definitely deserved the nod against Vito. I think Watts got a hometown decision. The Leonard fight could have gone either way (I had it a draw). Assuming the other draw Hagler had was fair, and there were no gifts he got in those obscure unfilmed fights, I think Hagler lost ONE out of 67 fights. That's pretty amazing, especially given the opposition and how he had to take the hard road up, fighting in relative obscurity, and going into the Lion's Den in Philly and trying to build his name against those quality philly fighters. Whitaker is a multi-divisional champ who is also one of the greats in a traditional division. He's often remembered for his great defense, but his offense gets underrated. He had one of the best jabs ever. He had an excellent workrate, terrific bodypuncher, good combos. Not a big puncher, but he could do real damage at times with his left hand due to timing, accuracy, or simply getting more leverage on it. He was on the downside of his career and arguably beat an undefeated, prime De La Hoya, an excellent fighter whom despite going up in weight, was actually a naturally bigger guy, frame-wise and a 4 inch advantage in height and reach. I'll lean towards Whitaker here. I'll say he was the best of them at his peak, and although he had considerably less pro fights than the other 2, he still achieved a lot. As said before, he's an ATG in a traditional division AND a 4 division champ, and it's hard to pick against him H2H at 135-140. Duran would be a slight favorite. Don't know who else.
Marvin Hagler and Tiger Flowers stick out to me as the top guys. (Nobody brought up the man that beat Harry Greb twice huh?)