Most famous what?
RedHat is (in the English speaking world) probably the best known Linux variant (Suse I believe is big in Continental Europe).
It all depends what you want -- it is most sensible to get certified in whatever you know or do.
If you are an AIX administrator, it is probably not wise to do Solaris or RedHat certification because it is better known,
you should get AIX certified (first at least).
If you dont know or use UNIX then you should question you motivation -- you can get Linux (many flavours) or Solaris for Intel free, so install one, get a book, study it and sit the exam.
If you want to know which UNIX to learn, then the answer is simple: learn the Korn shell, learn vi, and the rest will follow
James