Very interesting topic here. My pick -
Nadel
Reasons
1) King of Clay Courts, always beat Federer on them, Federer never beat him on Clay courts till date.
2) Nadel beat Federer on grass courts as well.
3) Nadel is a pure tennis genius, Federer on the other side slowly kills the confidence in you through his excellent game play, Nadel on the other side, shows his class with just-cannot-reach returns on any types of courts.
4) Nadel runs, Nadel runs like nuts on the courts unlike Federer who tries his best to let not the other player put him in a inconveninet position and yes atleast all the tennis i've seen specially all the finals where these two guys meet, Nadel protects his side of the boundary hitting back everything on the baseline no matter where the ball is actually. He runs effectively and displays true sportsman spirit ( Note : I didnt say Federer does not display it
)
5) Every player wants to be Federer but most importantly every aspiring player wants to fight like Nadel.
Enough said, i have justified my choice
Looking forward to some meaningful discussion here on.