I think Federer has a good command of the language, uses a wide varierity of colorful expressions and doesn't make many mistakes even though sometimes he overuses phrasal verbs and tries to sound too cool, instead of choosing more common expressions. I'm not native myself but I can't help but think he doesn't speak absolute perfect English (apart from his slight foreign accent). All in all, I think he would be my pick for the most fluent speaker. On the other hand, Nadal and most of the Spaniards are just bad at English, using poor grammar and wrong selection of words. I think Ferrer would be my pick for the worst English speaker. What do you native speakers think?