I've been using Bay Photo for a couple years and they've have been excellent.
Prices I'd say are pretty competitive for the quality. They may not be the absolute cheapest but they're the cheapest I've found for pro quality. Print quality is great. Packaging is excellent. They process quickly, usually a day or two. No minimum orders. They also do higher end stuff like exhibition, cotton rag, Fuji Pearl and a ton of other products. They can drop ship with blank packaging and no invoice. Shipper even damaged a package and Bay shipped another print no questions asked. Highly recommend them.
I've also tried a few others that do high quality work but have some quirks. WHCC has minimum orders policy. Adoramapix is closed for a lot of holidays. Mpix is pricey from what I remember.
Hope that helps.