If the best player is the most likely to get a team the most wins, I'd say the answer is clearly Tim Duncan. Every year his team has been above 50 wins, excluding the lockout season and he's got 4 rings.

However, if you assume the "best" player is the one with the best all-around skill set, Kobe undoubtedly the clear answer. Lebron will be far behind Kobe until he develops a reliable mid-range jump shot. That's the difference maker when it comes to SG/SF superstars- it transformed Jordan into the greatest ever and it's what makes Kobe the hardest player to defend in the NBA.