I'd say Michael Phelps.
To (most of) those that follow swimming beyond one week every four years, Michael Phelps could have quit the sport before the World Champs two months ago and still been considered the greatest swimmer, if not the greatest athlete, to ever live. Heading into the meet, Phelps held individual world records in his three best events, the 200 fly, 200 IM, and 400 IM. Holding a world record in an IM, specifically the 400 IM, already displays versatility (as opposed to, say, the other extreme of being the best in the 50 free and never swimming anything else). Holding another record in a non-IM event shows dominance in a specific stroke as well.
In Melbourne, Phelps
-led off the winning 4x100 free relay, swimming a time in the 100 free that would have won the event later in the meet, an event that is absolutely not Phelps' best event.
-broke Ian Thorpe's world record in the 200 free, becoming the first person to swim the event under 1:44, and winning the race by 2.42 seconds. Other than Mark Spitz, Ian Thorpe is the only other swimmer who some people felt was better than Phelps. Phelps, in his fourth or fifth best event, is now faster than Thorpe was in Thorpe's best (or second best) event.
-lowered his own world record in the 200 fly, winning by 3.04 seconds. His world record is now 2.47 seconds faster than anyone else has ever swum the event.
-lowered his own world record in the 200 IM, winning by 1.21 seconds. His world record is now 1.13 seconds faster than anyone else has ever swum the event.
-lowered his own world record in the 400 IM, winning by 3.52 seconds. His world record is now 3.41 seconds faster than anyone else has ever swum the event.
-won the 100 fly, beating the current world record holder, Ian Crocker.
He's even the third fastest person ever to swim the 200 back.
In swimming, people do not win 200s by 2 or 3 seconds, and they certainly don't hold world records in them by that much. In a longer event, like the 800 or 1500 free, it's not quite as unheard of. In a 200, it's absurd.
Markus Rogan, a world-class backstroker and Olympic silver medalist, said "People kept saying Thorpe at his peak was better than Phelps at his peak. Now that question is answered. I think he really thrives on challenges like that."
Massimiliano Rosolino, an Olympic gold medalist, called Phelps' 200 free "unbelievable," stating that he "couldn't [swim Phelps' time] even splitting it up and resting between laps."
Mark Spitz himself said, on the possibility of Phelps winning seven golds in 2004, that "all things being equal, if we could simulate the same scenario, [Phelps] has a lot more difficult task. He's elected to swim six individual events, as opposed to what I elected to do, which was four."