What is the best Atlanta suburb to live in?

I think you have to define "suburb." How far out do you want to live, or how close do you want to live. As Matts answered, Sandy Springs is great as is Dunwoody, both flanking the Perimeter (Mall) area. But you also have neighborhoods like Brookhaven, the Briarcliff/Sagamore Hills area which are "burbs" but inside I-285. Or you can go out as far as Cumming and Alpharetta. Forsyth County has some of the best schools in the area if you have kids.