I can only pick one? I live in South Texas, but I don't have any "favorites" here, just ones that I prefer. My favorites are:
Ryco trim in Rhode Island, because it's in an old mill and has pretty much everything you could ever want, for great prices.
Colchester Mill in CT, because...ok I have a thing for old buildings. But the ladies that work there are terribly friendly, always helped with whatever I was stuck on and one time I was lusting after a particular fabric line that was coming soon and they ordered it.
Fabric Mart on Oahu, the one in Aiea because I liked the enormous warehouse that was full of fabric, the huge clearance area and the great prices on everything that wasn't on sale.
Kaimuki Dry Goods on Oahu because they had loads of local fabrics.