I have used many and have been happy with all with each order. I have switched around for certain kits (when I did kits), finding a store that had all the ingredients for a recipe, and finding organic malt.
My two current OHBS stores are:
Barley & Vine, primarily because it is almost local. Online, B&V sells grains by the ounce. However, by email, B&V will sell grains in smaller fractions including to the gram. IOW, they will match a scaled down recipe. OTOH, they don't have the market to offer organic. I have not tried their kits, which seem to have a good reputation locally, but I discovered B&V after I stopped using kits.
Northern Brewer is one of three OHBS that carries organic LME that I know of. They seem to have about the widest inventory. Their kits are very good.
Others:
Maltose Express has superb kits, most of which are from their books Clone Brews and Beer Captured, plus many "new beer recipes" that have not yet been published in a book.
Austin Homebrew, mention in posts above, has probably the largest collection of kits, many hundreds of very good styles of their own and at least 1000 clones.
Seven Bridges Cooperative the most extensive organic store.
Brew Brothers grains to 1/10 pound.
Boulder Fermentation Supply very good source for gluten free malts.