Hmm, where to start (again)? Well, as far as the rings argument goes (since that's what most of the pro-Duncan camp is relying on), what Spurs le team would lose the le if you replaced Duncan with Hakeem? Be honest now. Yes, Duncan does have more rings than Hakeem. He has also played with better coaching and more talent for the majority of his career. He, Shaq and Magic are a few players that came to stacked squads and never really had to play on teams with piss poor surrounding talent. That wasn't the case from the centers from Hakeem's era (Ewing & Robinson). But if you want to stick with the rings argument go ahead....but both players have clearly shown they can lead a team to a le if you give them the right surrounding talent.
I will focus on the things Duncan has shown me he can't do. They are very minor things, but it's what seperates Shaq and Hakeem from him IMO. Really, it's only two things.
Offensively:
2001 - Horace Grant/Robert Horry (48%)
2002 - Robert Horry/Samaki Walker (42%)
2004 - Over the hill Karl Malone (47%)
2008 - Gasol (43%)
In each of those seasons, those teams (mainly Phil Jackson..who doesn't like to double) allowed the above players to basically play Duncan straight up...and dared Tim to punish them enough to get the win. And he couldn't do it (his % is in the parenthesis). Last years series against LA was a perfect example....Duncan just doesn't have that extra umph in his game to give you 35-40 a night in a series if you need him to. When he has to become a volume shooter and really pump points, he can't. He just has always had the luxury of not having to do that due to his surrounding talent. He can't hit that turbodrive...Hakeem and Shaq could. There is no way in you could rely on Gasol, Grant, Horry, etc to check Hakeem or Shaq straight up and get away with it. They would murder them. You can't tell me a player is in the same realm offensively as Hakeem or Shaq when they can be neutralized in the postseason by Pau freaking Gasol.
Additionally, Duncan shot 42% against Detroit in the 05 Finals, but still got the ring due to others like Horry and Manu stepping up (not saying Duncan didn't do his part..he was still the leader on both sides of the ball). But Hakeem never had the luxury of winning a series while he shot 42%. Deny all you want but surrounding talent has a lot do do with les. That's why I'm not so big into the ring argument, unless it's two dudes going H2H in the playoffs on teams that are just as good, and one guy is stepping up while another isn't (i.e. 95 WCF, Finals between Bulls and Jazz/Suns, etc).
Defensively:
It really isn't a compe ion on this side of the ball. Hakeem was clearly better and it's not even close. The dude is #1 in blocks and top 10 in steals. I have seen Amare kill Duncan in a playoff series. I have seen Dirk kill Duncan in a playoff series. Thank God for Robinson because Duncan never had the ability to check Shaq (before he got all old). Earlier on I noted Hakeem would win a le taking Duncan's place on any Spurs squad. I don't know if Duncan could have led the Rockets to any le in 94 or 95. Why....because he would have had to neutralize Ewing, Robinson and Shaq. And if he can't stop Amare or Dirk....then I'm not so sure how he would fare in those situations. Duncan is a great defender but Hakeem he is not.