We go to Tortola to shop about once a month, and usually wind up going to all 3 stores. To answer your specific questions:
Best selection: None of them. They each have different stock. Riteway is the largest, but they will fill a large expanse of shelf with the same item. OneMart and Bobby's have a lot of things that Riteway doesn't carry, but in much smaller quantities.
Best fresh produce: OneMart's produce is usually fresher. Riteway has a larger selection.
Best fresh poultry/meat: Riteway by a long shot. For the very best meat, go to Steakation.
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