1. Garnett
2. Duncan
3. Malone
Next tier holds many greats, including Petit, Barkley, Mchale, Dirk, and possibly a few others.
Garnett and Duncan are very close, but I think Garnett peaked a bit higher. Malone was awesome as well, but a step behind the other two due to their awesome defensive abilities. Malone was a good defensive 1-on-1 player, in the mold of but not as good as Mchale. Mchale was an excellent 1-on-1 defender of several positions.
But Garnett and Duncan have demonstrated an ability to anchor a team defense unprecedented for the power forward position. To not just hold down their individual assignment, but to also be dominant help defenders and rebounders as well. It's supposed to be the center that anchors the defense, but both KG and Duncan did it from the 4 (depending, of course, on how you characterize Duncan. Here, I'm lumping him with other PFs and not centers).
KG was unique in that he also demonstrated a similar team-impact effect on offense as well. While Malone and Barkley were better scorers, KG's offensive impact was much larger than his gross scoring numbers due to his ability to pass, initiate the offense, space the floor, or just pick-the-heck-out-of-everyone. It's why his team's tend to look so much stronger offensively with him on the court.
If your criteria is based on accolades, obviously Duncan with his 4 rings trumps the competition. But based on who impacted games more, these would be the top of my rankings