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.
Tier1(goat): Federer; Tier 1.5: Laver, Gonzales, Sampras, Nadal, Borg;
Tier 2: Lendl, Connors, Rosewall, Tilden; Tier 2.5: McEnroe, Agassi