You can't be serious. Sure, Agassi, Rafter, and Courier were all rivals with Sampras. But McEnroe?!? Wrong era... They played 3 times total and never on clay. And Lendl? Or Edberg? Sampras never played them on clay either. Becker? Sampras played him on clay only once (in Rome) and beat him.
Yeah you're right. Nadal, Djokovic, and Murray are way better competition than Agassi, Rafter, and Courier. It's not even close.
Sampras actually never played Kuerten on clay, beat Muster the one and only time they played on clay (which was at the French), and went 1-1 with Brugera the only 2 times they ever played on clay (which was also at the French).
I'm a huge Rafa fan and don't find myself defending Fed much, but this statement is just outrageous. (I'm also a Sampras over Agassi guy, but can accept that for as amazing Sampras was on grass and hard, he was an terrible clay courter). Fed has 1 title and 4 runner ups at the French. Sampras got to the semifinals of the French just once. Federer is probably the 2nd best clay courter of his generation (though Djokovic is making a case for that spot now) and would have won multiple times at Roland Garros if Rafa wasn't around. Sampras couldn't tie his shoe laces on clay. Federer's clay court results are light years ahead of Sampras's and the numbers support that fact.
p.s.
http://www.stevegtennis.com is an awesome resource for H2H records.