Of the summer Olympic sports and taken as group, runners have the least body fat. Nothing else puts such a premium on minimizing nonperformance weight and is so stressing cardiovascularly. There is a reason volleyball players run for conditioning, and runners play volleyball for fun. Even non-elite male runners shoot for below 6% body fat. I can’t imagine there is an elite runner that is more than 3-4%. (FYI, 2% of your weight should be internal, protect-the-organs-from-damage, fat, which is what ultrafilter is referring to.)
However, all the Olympic athletes are in absolutely great shape. Michael Phelps spends something like 7 hours a day in the pool, and yet he lifts weights and runs, etc. I have friends who work for USA Swimming, and they tell me that he ate 6500 calories in one meal with them. There isn’t an ounce of spare fat on him, and I’m sure he is in the 3-4% range, also. And so must be about every other Olympic male swimmer, and just about every other male athlete.
There are two reasons divers are fit. One is that they must work out hard too, to not get injured and perform their manuevers. The other is that diving is a “presentation” sport - looking good doing it is almost as important as doing it. A smooth, flowing diver with what gymnasts call “long lines”, will outscore a mechanical diver, or a diver with, uh, “thick[?] lines”. (Bodybuilding would be another presentation sport. If extremely low body fat were intrinsically linked to extreme muscle development, then someone would have mentioned the weightlifters in this thread.)
Now as far as best looking, I vote for beach volleyball and swimmers. I hope my wife also votes for swimmers, 'cause I’m too tall and rangy to ever resemble a gymnast. Of course, for all I know, all the archers and rowers are totally hot. We never get to see them.