Who is the Best Center in the NBA?

Who do you think is the best center in the NBA? I have trimmed the list down to four worthy candidates.

Dwight Howard 20.9 PPG, 14,2 REBS

Al Jefferson 23.1 PPG, 11 REBS

Yao Ming 19.8PPG, 9.6 REBS

Shaquille O'Neal 17.4 PPG, 8.8 REBS