To answer your question it might help if we had a little more information.
1) whereabouts in the world are you?
2) is there a particular network you want to use
3) contract, sim free or used?
4) what is your rough budget?
5) what do you want to do with the phone, games, facebook, photography, music, web browsing.... something else?
6) do you care about the size of the device
7) are there any features that you must/mustn't have?