...and it's still raining...

Federer has the best Federer? What?

Does he have the best *something so sacred, we dare not say what it is, for that would be blaspheme*?

Just kidding... Part of me is guessing you're talking about his forehand, or maybe backhand (in which I'd say movement, spin, and placement), or strokes in general (but that wouldn't be "one").

As far as groundstrokes go... Safin is scary because he looks like he's casually swinging a ragdoll or something and *BOOM* the ball goes flying over the net, in the court, and practically ripping a new one into whatever gets in its way.