Jeez, McHale is being underrated here, in my opinion. I wasn't alive when he played, but from what I've read, and from what I've watched, the guy was a monster on the block, arguably the best post player ever. If his feet don't give out, the guy probably could've played into his forties.
Duncan is better than KG. For starters, he has more rings. Second, he could play any style. He won two titles during the era when defense beats offense, he won another title during the era where the league was adjusting to new rules, and he won another when the league became offense, small ball oriented. Even today, he is performing at an incredibly high on a team that has consistently won year after year after year. Finally, he could adapt to any game or situation. If the Spurs needed 30 points, he'd do it. If they needed 20 rebounds, he'd do it. If they needed him to play defense and do the dirty work, he'd do it. I don't think the same could be said for KG (just personal opinion).
KG is all-time great. I love the guy, and he probably became the first guy who was an elite player that was a jump shooting big. I'd go as far to say there's no Kevin Durant game today if it weren't for KG. His defense and intensity are characteristics I'll always appreciate.
Malone is one of those guys who did great things, but in the end left you wanting more. I'm a firm believer that if he wasn't so scared in the clutch, those Jazz teams could have knocked off the Bulls. He was the best running big man ever, one of the most well conditioned players ever, and his PNR game with Stockton was one of the most unstoppable plays ever.
Nowitzki probably did the most globally out of all these PFs. After winning the title, his fame skyrocketed across the world, even after being loved worldwide, especially in Europe. I'd also venture to say his championship was the most improbable out of any of these PFs. Against the Blazers, everyone thought Dallas would choke, because they always did (and they collapsed in the Brandon Roy Game 4 masterpiece). But they pulled it out, and then SWEPT the defending champs, without HCA. That is impressive in itself. Then, Dirk single-handedly destroyed the Thunder, and capped off the playoff run by humiliating the Heat.
All that being said, my list goes like this:
Duncan
McHale
Garnett/Malone (it could really go either way)
Nowitzki