I have to go with Dirk here. Rarely a time when the player actually shows you how the team would fare without them, but Dallas was awful and went 2-7. You can argue that Butler was out, too, but the Mavs lost to Toronto at home with no Dirk and a healthy Caron! So, if you take Dirk's winning pct% this year, he is at about 81% winning, which puts them right below the Spurs current mark now.
Lebron's games out (0-2) are tougher to call since they lost @Denver and @Chicago. I just get the feeling that they'd be a playoff team with Wade and Bosh and an average NBA player where James is. I think Dallas would be a lottery team with an average PF where Dirk stands.
I do think that it is a 3 horse race. Lebron's obstacle in the discussion is his teammates, but that's what he signed up for. Rose's obstacle is his efficiency(54% TS) and I think the Bulls' overall defensive improvement has more to do with their jump than Rose's leap. I do think Rose still has a very good shot at it. I just can't stop looking at that 62% TS that Dirk has on a very high USG rate. I think that guys like Lebron and Rose have that spectacular plays streak in them and that ends up LOOKING better to the fans, but a basket is a basket.
Haven't even mentioned Dwight, but you can easily make a case that he is the most valuable in the league and could be the first pick in a leaguewide "start from scratch" fantasy draft.