If the criteria is greatest "athlete" then I disagree with Jordan. He tried baseball and failed. If you're looking for someone who is a great athlete then someone like Bo Jackson should be mentioned since he was great for a short time in football as well as baseball. I'm not suggesting Bo Jackson should be on the list because I don't think he would get a vote and people would just laugh. But his all-around athleticism can't be denied.
For that very reason I voted for Jim Thorpe as one of my choices. He was voted as the top athlete of the first half of the 20th century (back in 1952) and that alone should be considered. He played pro baseball and football and was an Olympic star and probably had more athletic skills (all around) than anyone since.
There have been some really good ideas exchanged here. Thanks to everyone who put in their 2ยข.