Not saying he's the best ever yet, but I think he's at least starting to enter the conversation. He's 3 behind Nadal and 6 behind Federer. You have to think he'll get at least 2 more now this year. Then you look at competition and he's winning many of these in the Federer-Nadal-Murray era. I love Federer to death but he won a lot of his majors in between the Sampras/Agassi and Nadal/Djokovic eras.
Step 1: He needs to win the French. Almost did it the last couple years. Needs to do it this year. Gonna be 29 this summer so you have to assume he's got about 2 years of playing in his prime left before the tires start treading. If he were to get to 15 plus the French, he'd be above Feds in my book (of course assuming Feds doesn't win any more).