Most of the other answers seem to imply computer-based rpg's are the default but I suspect you refer to table-top.
If that's the case, there's an argument to make for several:
Dungeons & Dragons - in all its various versions, is definitely the most popular, well known, and successful. There are hundreds of A vs B comparisons with Dungeons and Dragons coming out on top quite often.
Gurps, by Steve Jackson Games, is an amazing sandbox style universal system that gives great material for building exactly the style of campaign you wish, no matter how eclectic that campaign may be. Unfortunately, it is definitely a planning and creativity heavy system requiring a lot of thought be put into campaign development and character development throughout the process of play.
Pathfinder, by Paizo, is a spin off of the 3.5 edition of Dungeons and Dragons. Several modifications streamlined the system but it is still essentially d&d. It has the advantages of exposure to the iconic fantasy tropes common through its parent system while still being different enough to appease folks who feel D&D is too commercial and moving away from the art.
FATE, another universal system -- by Evil Hat Productions, has many of the advantages of open ended creative design in Gurps but leans much more to the artist rather than the simulation. Much of the FATE system leans towards development of the literary or film style campaign, including concepts such as dramatic influence and plot manipulation.
All of these systems are excellent choices for 'the best rpg' when referring to table-top roleplaying. Of course, there are hundreds more that could be considered 'best' because of specific characteristics.