That being said, I'm not sure Ainge is the "best" or that a title puts him over the top.
w/r/t LAL - hasn't there been some turnover? Kobe and Shaq were brought in by Jerry West. So the last run in the late 90s and early 2000s gets current GM no credit. That means he's on run 1, just like Ainge. But Kobe >> Pierce, so I think he's << Ainge if C's win this year.
Other guys would be Pop in SA (he's the real GM.) He's capitalized on having Duncan for his whole career - but that's not trivial. Minny couldn't do anything similar for KG, and had more opportunity to do so. Ainge has capitalized on the opportunity KG has presented, maintaining a core that could win now, adding pieces, and keeping future possibilities available. (Would you rather be in SAS or BOS position for the next 5 years? They are both top contenders this year and similar prospects for winning after this year with the core.)
Detriot? No way. He's made too many bad franchise killing deals. It appears he fell into a championship and his true colors are failure. (He's like the inverse of Ainge, who made some silly initial moves - Scal, Raef, but got better as he got his base.) Also look at his coaching turnover. He whiffed big on the Darko pick too.
Who else Riley in MIA? I think he gets less credit than he should: pulled off a quick Shaq steal to sneak in a title, then retooled on the fly to get Lebron and Bosh. That team will get better every year for the next couple years, and you have to guess they'll get a couple of titles. That would be two legit title contenders, and even though Wade is a solid piece, he brought in a better piece to build around.
You have a bunch of other "contenders" that don't impress me: ATL, ORL, CHI, PHO, DAL. Dallas has done good work to keep things competitive during Dirk's career, but there have been serious misteps (and who is the GM Cuban or Donny?)
Then you have a bunch of the young stats guys in POR, SEA/OKC, HOU - but really they are too new to know if their approach will yield better results. POR for one seems terrible at injuries - both evaluating and preventing, GM has to take some heat. HOU's team is a well positioned, but I don't see a title any time soon.
No one else jumps out. That was anecdotal.
I think you could look at what they inherited and what each move did to reposition them and if they made their team better or worse over that time to evaluate more objectively.