No, Mariano Rivera is not the best pitcher of all time, because he still has not eclipsed Nolan Ryan. Nolan Ryan accomplished far more in his time than Mariano Rivera has in his. It's hard to compare players from different eras, but Nolan Ryan was the Michael Jordan of his era, and has not been duplicated since.
You'd have to compare some sort of statistics to know that for sure and match them up against everyone who has ever pitched a ball in history. First off, there would be no way of getting statistics off everyone who has ever pitched, therefore saying "of all time" is an extremely tall order.