He has to be one of, if not the most clutch player I have ever seen. That's something that overall stats may not truly capture.
For example, "Mr. November", and then when he threw Giambi out at the plate on one of the most spectacular plays I have ever seen in my life in the playoffs where it was a ball that was thrown up the line in foul territory on the first base side, and he comes out of nowhere to redirect it to Posada to get the out. I guess I have to give him credit for the ball that Jeffrey Mair caught in 1996 that really should have been a ground rule double as well.
The fact is, he's won five World Championships, and the only other common denominators are Mariano Rivera, who is the greatest closer of all time, and Posada, a very solid MLB catcher in his prime.
Also, the Yankees haven't had very many captains. I think the guys that have been captains are people like Lou Gehrig, Thurman Munson, and I think Babe Ruth was for a short time, but he got into a fistfight with someone in the stands and lost his title. But that says a lot about his leadership abilities, which is something that a lot of baseball people overlook sometimes because it's not one of the five tools.