Fed has hit so many unique shots, but the one he keeps doing quite often and that I've never seen anyone else hit is the fake dropshot slice winner.
Not consistent enough to be a signature shot though. It's truly unique in his game (as is he smash smash winner and the hot dog) but he can't use it often enough to be called a signature shot. So with that being said, half volleys off any kind of shot his opponent plays it is. I mean a recent example is Nadull in the wind in that IW SF. That was some sick BHs he was playing. Truly genius.