Depends what you want. Valley Farms is the cheapest, Ideal has the smallest minimum order, My Pet Chicken is known for having a large selection (but I've heard that it's not an actual hatchery, more of a retail service; its stock is from Murray McMurray, I think? Or maybe Meyer?) Cackle is said to have the nicest birds.
And those are just the big names of the major backyard chicken distributors. There are all sorts of smaller breeders such as Greenfire Farms and Sandhill Preservation that focus on rare/show quality chickens.
Go with the one closest to you. Be sure that you don't have them shipped around a mail holiday. I would also have them shipped earlier in the week. You will need to be available to pick them up at your post office.