Without a doubt it is Shaquille O'Neal. Especially in his 3 year championship run in LA. The guy was impossible to defend, a very underated passer, and always cam through in the playoffs. Olajuwon did outplay him in the Finals, but that was when Shaq was young. I remember a few years later when Shaq just destoyed the Dream, and Hakeem said he was 'easily the best center' he ever played against.
Second place is where it gets hard. It is a tough decision for me to decide between Olajuwon and Kareem. Now, I was only born in 1975, so I only really remember the last 10 years or so of Kareems carrer. The guy was amazing though. He had the greatest 'go-to shot' in the history of the NBA, and made countless huge shots for the Lakers. He wasn't a great defender, but was better then he was given credit for.
As good as Kareem was though, I'd have to give the nod to Hakeem (if it is only from when I saw them). I am sure if I saw Kareem in his early years then he would easily be second and maybe even 1st, however with just what I saw, 'The Dream', has to be the runner-up. Olajuwon is the most agile big I have ever seen, and the only guy who may be close is Amare. Olajuwon's young life in soccer gave him the ability to move his feet like a shooting guard. He was an incredible defender, and molded himself into an almost unstoppable offensive force. His post moves were jaw-dropping, and he was an absolute joy to watch play.
Honestly, these 3 are so far ahead of any of the other centers :notworthy I have seen that it is not even fair to list others with them. Honarble mention to Moses Malone (eventhough he was kind of a PF), David Robinson, Pat Ewing, and Robert Parrish (I am sure I am forgeting some players).