Center is best. Because more nodes are close to the base station. But, in real networks, it isn't practical. In most of works, researchers place it to outside of the network.

Thank you Orhan for your repsonse.

Center is best for 3 protocols Leach, leach-c and pegasis by using simulator NS2?i know that outside is best for leach.But i haven't knowledge about leach-c and pegasis.

Best regards

I'm using OMNeT++ Simulator. Look at the length of the green lines for that round, at first image. Most of rounds are same. It shows base station for leach and leach-c. And look at the length of blue lines for pegasis. 

Thank you very much Mr Orhan,

So, we can say that the center is the best position for the 3 protocols.is not it?

Short answer: Yes :)

Long version: For randomly distributed networks, network's center is best. Because more nodes are close to the base station. So, distances between CHs and BS will be minimum.

thank you Sir

All WSN papers that deal with clustering locate sink in the center. The locations of other nodes are random with normal distribution. this makes the center more near to most of the nodes. If you try to use K-mean algrithm to find four different clusters you will find that they located around the center.