Did you just say swimming is easier than running? I hardly think that is true.. In theory they should be equals. If you put everything you have into a swimming race, you should be working equally as hard as if you were doing the same in running.
As a swimmer, I look at the different events, even from within the same stroke, and I don't see it as similar at all. A 50m free is not the same as a 100m free. Pace is important, and so are turns. There is so much more.
Also, you have to look at the mastery of every stroke. People DREAM about being able to break records in every stroke. Phelps is the first person to have been able to show that he was capable of being a top competitor in every single stroke, and every single event.
Now, if you put him in a 1500m free, you would probably see different results. He is an amazing sprinter, and he is good at the widest variety of events. When you look at it objectively, and based on numbers, he is THE greatest olympic SWIMMER.
Is he the greatest olympian ever? That's a hard call to make.