In terms of what? If we're talking musical ability, then the Beatles hands down, but that isn't really fair, because they were a group and Elvis was a solo artist.
If we're talking influence, which I assume we are, then it's very difficult to say, as they are the two most popular artists in the history of rock and roll (literally). If you put a gun to my head and said I could only pick one, I would reluctantly say the Beatles again, if for no other reason than that they did not only revolutionize rock, but pop, and psychadelia as well. Not to mention songwriting. No matter how iconic Elvis might be, his music is nothing in comparision to the Beatles as far as complexity. However, if not for Elvis, who knows whether the Beatles would have ever gotten together.
But on the other hand, who says that rock and roll wouldn't have surfaced anyway under Buddy Holly and other musicians of the era?
It's a very complicated question.