Jim Thorpe is the best Athlete to ever play in a professional sport. Obviously, athletes improve over time, but in terms of just accomplishment and scope, it's not really even close.
Olympic Decatholete and Pentathlete Gold Medalist, Professional Football Player, Professional Baseball player, Professional Basketball Player, Scratch Golfer. He never played tennis but there are stories of how good he could have been had he picked the game sooner, as well. He was just a fantastic athlete.
Now there are those that will point out that athletes today have eclipsed his records etc. but if you look at it that way, then the same argument can be made against MJ. Todays Athletes are better then MJ, as great as he was. There is no comparison because time moves on an athletes improve, that's just how it is, but in terms of just simply looking at what these athletes were able to accomplish, it's Jim Thorpe by a considerable margin IMO.