Shaq is(was?) a great center, but calling him the greatest player is not even close and downright stupid in my opinion. He is 34 and was almost invisible in the Finals in terms of stats, Kareem was able to play way longer than age 34 and was always a threat. There are more than a couple of people ahead of Shaq on the All time best list. I think Brick said it best -- Wilt, Russel, and Kareem were better than Shaq and Shaq never really had anyone dominant to square off (that played the C position) against, unlike the other players mentioned above, so its hard to judge him.
Its a tough comparison, its the chicken and egg story. You say that there were no other dominant centers to square off, but maybe he was way better than all of them that we get the perception that there were no other centers in that time.
Some of the centers that he went against were really world class and may not be the all time greats but some one I would love to have on my team during their prime.
Hakeem (though fading away still was a force)
David Robinson
Patrick Ewing
Arvydas Sabonis
Vlade Divac
Rik Smits
Zydrunas I
Tim Duncan (yes he is a 7 footer, and is a center its just that he hides behind another 7 footer in Pops scheme and they did go head to head many a times in various games )
Alonzo Mourning
Not a bad list, maybe not all time greats but a decent list anyways.
On the other hand, he may not be a candidate for the GOAT just because his dominance did not last that long, it was only that three years when the lakers won. Maybe he was dominant but he didnt get the job done the rest of the years. Whereas players like jordan, magic etc found a way to win and maybe were lucky to be part of a team that managed to win way more than Shaq did.
Even in this recent ring, he was as dominant and effective in the game as Zo was, in fact i felt that the heat were much better off when Zo was in rather than shaq in. Sure enough Zo was competing against the bench when he came in and Shaq just being on the floor opens up for players like Wade to win the mvp. I have to give it to Shaq on one thing though, all those great players played in a time when the rules were not so against them, with the zone rules it is easier to deny the ball to shaq. Its easier to defend great players and it becomes tough only when there are multiple threats on the floor.
If the rules were the same during jordans years maybe jordan doesnt get the ball and when he gets it the other teams might put two on him with a spy towards the basket and another one staying with pippen. We wouldnt know the result.