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.
Roger is number one - 17 slams; 302 weeks #1, 6 WTF; 5 years end #1; 24 slam finals;4 slam finals back-to-back year; Player of the decade