Federer on paper has the hardest group with Murray (a guy who's got his number 6-3), Del Potro (Slam winner and beat Federer in their most important meeting) and Verdasco (AO semi finalist and should have won the second set in IW).
But in reality and going by who's hot currently, then Nadal. He's got Soderling (the guy who beat him at the FO for the first time), Djokovic (thrashes him at nearly every opportunity on HC) and Davydenko (clinically beat him when they met in HC finals).
The thing is, Federer wouldn't mind Nadal's draw with his pigeons Davy and Sod in it who he has a whopping 24-0 record against (or something like that). Even Djokovic is his pigeon, but I'm not sure that means much these days.