How did this thread become about the Fed / Nadal rivalry?
OP asked about Rafa / Djoker vs Sampras / Agassi, which is a valid question.
Anyway, Sampras / Agassi rivalry is far superior for a couple of reasons:
1. Contrasting styles: great server vs great returner, S&V vs aggressive baseliner, etc. Rafa and Djokovic are both baseline grinders (ok, maybe grinders is too harsh, but you know what I mean - they are more alike than different in style).
2. Competitive on most popular surface: Sampras and Agassi played most of their matches on hard courts, where they have a close to even record. Contrast with Nadal / Djokovic, where they have played a lot on both clay and hard, and Nadal owns Djokovic on clay, and Djokovic owns Nadal on hard. So pretty one-sided, with which side just depending on the surface.