Djokovic - Backhand on full stretch. No doubt it's his most brilliant shot. Very unique shot imo. I don't even know how many times he pulled off sick BH winners on full stretch. His balance on the BH wing is just unreal.
Nadal - The banana passing shot - Extremely unique shot. No one can do it as good as Nadal. Everyone remembers Nadal whenever they see anyone playing a decent banana shot.
Federer - Maybe half volley?! Well I'll let his fans decide this. He has many great shots so I'm not sure which one is his signature shot.
Murray - Drop shot. His drop shot looks really unique to me and very effective at the same time. Even Andy himself said it's his fav shot.
Del Potro - Super flat Forehand CC at full power - http://www.youtube.com/watch?v=7RcxYFBOehU.
This shot to me shows Delpo's full potential.
Tsonga - Stop volley/Drop volley - Just go watch his match vs Nadal in AO 2008. Some SICK drop/stop volleying from Jo in that day.
Ferrer - Inside out FH - It's certainly his best shot but there are many who have a better inside out FH than him.
Berdych - Not sure which shot is his signature shot. Maybe FH inside out.
Tipsarevic - Backhand DTL - His BH DTL is top 3 at the very least at the moment.
I will leave the other players to you guys.