My 1st choice would be Shaq. He's basically the only good center left in the NBA, and no one can stop him. The only thing that worries me is that his age is creeping up with him, but he still has another 3 to 5 years left to dominate the league.
Tim Duncan would be my other choice. He's already won a championship, young, good attitude, great all around game, and a team player. After the Admiral retires, Duncan will probably move to center like he did at Wake Forest.
There are too many shooting guards in the league to choose to build a team around.