Since the word athletics truly refers to track and field, then that is the sport. The Olympic decathlon winner is referred to as the world's best athlete.
In a more typical sense, it all depends on exactly how you define "most athletic". I've heard that soccer players can run 11 or 12 miles in a match (90 minute) but this pales in comparison to marathon and half-marathon runners, who also get no breaks.
Some sports are also faster paced than others. For instance, a round of golf can take the professionals near 6 hours to make roughly 72 shots. This is only one shot per 5 minutes. Basketball and volleyball are much faster paced.