Wow, that's like asking "what's the best chocolate?" :shock: You have to try them all in order to decide what's the best one, right? :lol:
In addition to several that have already been mentioned, I have had great results with the following shops:
Craft Connection
http://www.craftconn.com/
Erica's
http://www.ericas.com/fabric/
Fat Quarter Shop
http://www.fatquartershop.com/
Havasu Quilts
http://www.havasuquilts.com/