.  Champions always secretly believe they are the best, you have to, it's part of the mental makeup.  Sampras in his prime said he believed he was "Unbeatable", this is necessary to be great. Without believieng that you may as well throw in the towel before the match.  Overall? They're close. Sampras had 0 more slams, a bunch of WTF and a lot more weeks at #0 (logical since hard is the majority surface, clay the minority). Nadal has the golden career slam and a lot more master titles. He's also the #0 on clay, something that Sampras is not on any surface. So it's a toss and it depends on which criteria you value the most.  