If we're going to simply talk about strokes and leave out every aspects of the game from the baseline then I would say Safin. At their best, Safin beat Agassi. His power makes the difference. He was the one who could sneak a win over prime Federer in 2005. Agassi never beat Fed again at the end of 2003.
NadalAgassi: I think Serena's final slam tally will be something from 18-27. My best guess is 24 or 25 though; Nole(2010) will never win Wimbledon