No one else has won that many events. If you want to count Jim Thorpe he won lots of events against other people who weren't specialists in those events. Every event that Phelps lined up for was against the best in the world in that particular event, rather than the best over a range of 5 or 10 events. Swimming that many events is fairly similar to a runner competing in and winning all of say 100m, 110m hurdles, 200m, 400m, 400m hurdles and a couple of relays. Added in the hurdles here to account for the different style of runner, since Phelps competes in different strokes (which most swimmers don't).
May not be the greatest "Athlete" as that is VERY subjective and depends entirely on what you think is the definition of an athlete, but what he has done is absolutely amazing. I say this and I'm not an American.
