Rafa is the best in several areas, volleying isn't one of them, not even among the top 4. Look at how Roger used the net consistently and with devastating effect in an event as mammoth as the Wimbledon Final 2012, against Andy Murray. There's little comparison. I agree with others as well that Nole is more willing to use the net than Nadal though Nadal's volleys may *possibly* be marginally better.
My favourite players is Roger Federer. Rafael Nadal historically has the best mental fortitude, stamina, shots on the absolute full run, defence and possibly speed of the current top 4. I don't pick Roger's volleys over the others because he's my fave player, I pick them because *they are in profound likelihood the best*.
Federer:
Forehand
Footwork
Offence
Volleys
Serve
Drop-shot
Slice
Nadal:
Mental fortitude
Stamina
Passing shots
Defence
Speed
Stroke consistency
Top spin
Djokovic:
Return
Lobs
Flexibility/recovery
Changing direction of the ball
Backhand
Murray:
Cat and mouse exchanges (Throwing him a bone)
I'm probably forgetting some things.