It's hard to name just one.

Earl Campbell wasn't the best football player ever, but I haven't seen anyone surpass him in his freakish combination of speed and power. Bo Jackson and Herschel Walker are close. Also, for pure athleticism Rod Woodson was an unbelievable athlete and one of the greatest defensive backs ever.

I agree with SC on Babe Ruth. If he had not converted to playing the outfield, he would have undoubtedly gone down as one of the greatest pitchers ever. The gap between his talent and the rest of the players during his time was far greater than that of any superstar in any sport, including Wayne Gretsky and Michael Jordan.

Jim Thorpe, as SC mentioned, played three team sports very well and was an olympic legend.