Jerry Rice is considered by almost everyone to be the greatest receiver of all-time and with good reason. No one comes anywhere close to the numbers he put up in his career. His consistency over a long period of time is really unbelievable.
The argument for Randy Moss is the impact that he had on games. Early in his career there was not a defense that could stop him, and he was capable of "taking the top off the defense" on any given play. Rice was never that dynamic.
Mike Tanier (formerly of Football Outsiders and now of Sports on Earth) used some FO stats to compare the careers of Rice and Moss. Tanier does a great job of breaking down those numbers, so go over there and read that.
While Moss is my favorite player of all time not named Steve McNair, there really isn't much of a case for him being a better receiver than Rice over the course of his career- and that is what it takes to be considered the greatest of all time.
However, Moss is, without question, the most dynamic receiver of all-time. He also is the subject of the greatest YouTube video of all time.