Murray wouldn't have been my first choice anyway, but:
Federer. I just appreciate his game more than the others'.
I think nationality is largely irrelevant in tennis: the players are individuals, generally not playing for their country as such, so why should I select based on nationality? Sure, it's nice if a Brit (note I left out the word "plucky", despite not seeing most of them anywhere but Wimbledon
) does well, but I won't necessarily support them if they're playing someone I like more.