trying to say anyone is the best in any sport of all time is hard and difficult to do and has way too many variables involved: however.....
personally i put federer and sampras as co-best of all time.
in reality you can't put the number of grand slam titles as the only proof as the best of all time. for instance.
rod laver, who won the grand slam in 1962 and again in 1969 (TWICE!) is for one thing only one of two men who have won the calendar year grand slam in tennis history, the other being don budge, and of course he (laver) won the grand slame twice!
what commentators fail to say (including all media) is that from 1963 until 1968, laver couldn't play ANY GRAND SLAM TOURNAMENTS! because he had turned pro and pros were not allowed to play them until the open era of tennis started in 1968, so can you imagine how many grand slam titles he would have won if he was allowed to compete those 5 years during his prime? and he still had 11, only 4 short of the new record!
and laver's competition was very tough both as an amateur and a pro. then again....
back then 3 of the 4 majors (aussie, wimbledon and u.s.) were played on grass, so does this limit just how all around good laver was or wasn't. i mean a grass court expert who played like crap on any other surface could have an awesome record in grand slam titles?
for instance in regard to nadal, what if now, 3 of the 4 slams were on clay? or if that was the case back in the day when borg dominated on clay like nadal does? speaking of nadal....
personally i don't think he will have that much longer of a career, if he is constantly breaking down physically like he does already, but it's also his style of play that is so griding on him...
as far as head to head against federer, yes, he has a comfortable head to head record against federer, however, most people fail to mentioned even the so called tennis experts a.ka. mary carillo, mcenroe, etcetera, is that most of their head to head matches have been lopsided by playing on one surface way more than any other surface and that surface being clay! which nadal along with borg is already considered the best of all time on that one surface.
hardly indicative of a what i would call a true head to head mastery over federer, although nadal has beated federer on hard courts (in january at the aussie open) and of course last year on grass at wimbledon, by and large most of their matches that nadal has won have been on clay, although federer has also beaten nadal on clay i believe twice. the latest one just a week before the french open.
i believe the head to head with federer and nadal would be quite different in roger's favor if they played mostly on grass, indoor and hard courts.
in regard to sampras, he had infinitely more and tougher competition to go thru than federer. sampras had to go thru other grand slam champions, whereas federer has to content with besides nadal, only roddick (1 time grand slam winner), murray whose rising fast, and djokovic.
sampras competed against becker, edberg, courier, chang, agassi, lendle, a past his prime mcenroe, and more. plus different styles including the now almost defunct serve and volley game. which now with the advancement of racket technology improving in power every year even since sampras's recent retirement makes today's game like the women's game, power groundstroke bashing and that's it. federer of course being one of the few who can do it all.
i think prime sampras would barely beat prime federer on grass, on indoor court and hard court it's a toss up and federer tops sampras on clay. i think people think that federer's game is more all court that sampras and the shots that federer can come up with are amazing and genius and that's why some think he's better than sampras. lastly, would love to see federer play in the 80's to mid 90's with a less technological racket and play against serve and volleyers!!!