I'm always uncomfortable with question like this. Not only is it almost impossible to pick one, but what exactly do we mean by "greatest"? Is it in size of the person's waistline?
In any case, Physics World is conducting a poll on on their Facebook page on who people would consider as the "greatest" living physicist. They proposed or nominated 5 names:
At least they included a condensed matter physicist!
You can cast your vote there if you wish. I'd rather ask you on who you think should have been included for consideration on that list, and why.