Tough to say.. Numbers wise its Fed.. But Sampras and Connors have a say in this as well.
Both had better competition IMO throughout their primes. You can also take into account, Feds main HC opposition was Roddick for a long time. Good player, but even he was destroyed by an old Sampras and Agassi on the hards.
Nadal didn't really come into his own until 2009 or so as far as hardcourts were concerned.. Prior to that he couldn't even make a slam semis.