Conch Flats General Store, at Disney's Old Key West Resort, generally offers a variety of items. Sandy Cove, at Disney's Grand Floridian Resort & Spa, dedicates a small amount of wall space to DVC merchandise and the offerings are limited.
I think Aulani has the biggest selection overall but at WDW I personally like shopping in the store at AKV in Jambo House because their merchandise is usually a little more unique. But really, all of the DVC resort shops devote only a very small portion of rack space & shelves to DVC specific items.