I mean, who else? Cal broke Lou Gehrig's record for consecutive games that year, and you could make the case that it was an even rarer achievement than Phelps' winning eight gold medals in Beijing, but that's really not the issue here. Both reached the absolute pinnacle of their particular endeavors. Each displaced one of the legends of his sport. Both are beyond deserving of all the recognition and praise they have received in the aftermath.
There is one big difference, however. Michael didn't save the sport of swimming from itself. He certainly raised it to a new level, but it was in no danger of being eliminated from the Olympics or falling into disfavor with the general public when he began his historic medal quest.
That's why Cal is actually more deserving than Michael, and I say that with all due respect to both. Cal picked Major League Baseball up after the disastrous 1994 work stoppage that forced the cancellation of the playoffs and World Series. He ushered back a nation full of disaffected fans with his pursuit of Gehrig's record, even if the act of simply playing every day did not engender the kind of suspense that accompanied Michael's phenomenal eight-day run in China. Ripken spent the months leading up to the record-breaking game standing for countless hours at Camden Yards and around the country signing thousands and thousands of autographs. He did everything but go door-to-door to bring the fans back to the ballpark.
Both deserve every bit of SI's Sportsman of the Year award, but Cal deserves it a little bit more.