What is the best shop in Oblivion

It depends, but their isn't really a best shop. The various shops across Cyrodiil don't all carry the same items. To get the best price on a particular item, raise the shopkeeper's mood by bribing them or coercing with them (joke, brag, etc.). Common items (such as calipers) are easily found outside of a shop in any city. Check inside of boxes, barrels, and chests.