Whenever I see an article that says RF is Greatest of All Time (GOAT), there is always a qualifier "arguably". I don't know what else the guy can do. Anyway I myself don't believe in GOAT, but he is the best I have seen since I started watching tennis 25 years ago. There is also this constant comparison of him and Sampras, but I think that is a wrong comparison. The person he should be compared with is the great Ivan Lendl. In the chart below I show how Federer compares with Lendl, Sampras and his greatest rival, Rafael Nadal. Though i personally don't like Nadal kind of play, I like the intensity he brings in to the match.
In the chart below, winning percentage(# of matches won/total matches played) in a season is plotted vs the year (Year is normalized to reflect the total year they played as professional tennis players). Notice the longevity of Lendl at the top of his game, only partly matched by Federer. Federer's greatest years were undoubtedly 2004-2006 when he was always above 90%. Only Lendl has 5 years above 90%. Federer may not be able to match that, but he is the only player apart from Lendl to have lost the Year end # 1 ranking and regain it, Federer doing it this past year.
It is interesting to note that Sampras never had even a single year with winning % above 90. Clearly Federer is towards the end of career as per the historical data shown in the chart. I only hope Nadal curve doesn't fall of precipitously.
As somebody noted, since 2004 Australian Open, Federer has lost in a Grand Slam event only to the eventual winner. I hope he breaks the 286 weeks record as #1 currently held by Sampras. Of his 62 titles, 16 are GS and 16 are Masters.
I hope I can get to the US open this year to watch him play.