Federer's outfit (and hair style!) from 2005 is the best any man or woman player ever looked imho As far as women go i kinda liked Sharapova's black tight outfit she once wore! I don't remember when that happened, got to be one of her RG runs...Those, who play tennis video games should remember it from Grand Slam Tennis 2! Simple, Minimalistic and yet very elegant!
My favourite mens ones would be Federer USO 2005, and Rafa Australian Open and USO night outfits from 2009.
Womens... where to start. I loved the recent trend of girls in the short shorts. Vika was the one who seemed to start the trend a few years back, and more recently the Putintseva's and Ostapenko's have rocked it equally well.