Nalbandian is an underrated volleyer, no doubt.
Federer: his volleys are quite overrated too, as you could see today against Murray.
Djokovic: the fact that some people ever seriously considered him the return GOAT shows how incredibly overrated that shot of his was. I would also add his BH cross court (seriously, can't get much angle at all unless he's playing Nadal) but it looked great against Federer in RG 2012. Last but not least his movement, which Mats Trollander claimed was far ahead of Nadal.
Nadal: his returning probably.
Murray: his backhand can be unimaginative.