Well let's narrow it down
The Mavs/Blazers/Magic aren't really in the discussion
The Lakers you could make a small case for because of Bryant, Gasol, Artest, Odom and the defense of Fisher, Bynum, and Artest
But it comes down to the Cavs/Spurs/Celtics ultimately as the last three standing.
I see the Spurs as the weakest of the three. A prime Michael Finley, and Parker starting would make for a strong backcourt but Mo Williams is no slouch in his prime either. LeBron and Jefferson is really no contest. It would be much harder for Jefferson to get in a rhythm when guarding a player of that calibre. Tim Duncan and Shaq pretty much wash each other out but Shaq will get anybody in foul trouble and when Tim isn't in there, nobody will even slow him down.
The best two teams IMO are the Cavs/Celtics.
With a prime Shaq and a prime Lebron, they resemble the early 00 Lakers. The only difference is defense. The Lakers played incredible defense, and had tall physical bodies to throw at people while the Cavs really don't.
The Celtics have a bunch of guys they could throw at Shaq. Perkins, Garnett, Wallace, Davis. The talent on Boston would be just too much. A prime Sheed would eat up Varejao inside, while Garnett could take Shaq outside. LeBron would still get his, but the Celtics have bigger defenders that would make him work a little more than SA would