We love sports for two reasons: for love of the game and to become a champion. I love watching championships being won. I watch any championship game that's on, which means I watch lacrosse only once a year. There are two types of champions: the surprise team/player who just seazed the moment and the heavy favorite that lives up to potential.
Michael Phelps redefined what it means to live up to potential. I've heard plenty of people say how difficult it is to win when everyone expects you to. That covers teams like The Patriots, The Spurs and the Yankees of the last 10 years. What Phelps did was different. He destroyed the competition. It was Phelps vs the field, and you gotta take Phelps. Only few athletes have done that in my lifetime. Here are the athletes I put Phelps with(from my lifetime).
Pete Sampras/Roger Federer: You knew what the Grand Slams looked like before the year started. They would win the Australian, Wimbeldon and US Open. It was all about the French Open. They dominated in an individual sport that had hundreds of professionals aiming at them.
Michael Jordan: It took a whole team, but we all know that Michael Jordan essentially single handedly won 6 NBA championships, and if he hadn't retired it would have been 8 straight. But he did it during an era when it was predominantly a US game, not the worldwide game we see now.
Michael Phelps: He didn't lose. He found that extra gear to get the clutch victory. He set a record for gold medals when literally the whole world is challenging him. And he did it in a competition held only once every 4 years. And he will swim again in 2012. The next 30 years will help us understand how remarkable this really is.
Tiger Woods: In tennis you play one man each match. Basketball is about 12 with subs. Phelps swam against 7 at a time. Tiger Woods faces over a hundred each tournament and has dominated. He blows people away. He wins the close call. He wins hurt. I remember saying to myself during his "Tiger Slam" you can't just say "I'm gonna win" and then go do it! Phelps really only compares to Tiger. Both are men who have changed their sports for a nation. You can't give a bigger compliment than that.
EDIT: Lance Armstrong: How could I forget the 7 time champion following cancer? Probably because it's cycling. Doesn't always come to mind. Still, it is hard for any athlete to dominate for 4 years, let alone 7.