Nicholas's Fleet Street Shell has pretty good gas prices, however don't believe their discounted days (as far as I can tell the signs have no correlation with their actual gas prices. It seems to be a family owned business and they have a small convenience store inside. It's also a good place for oil changes and minor repairs. They may not always have your car ready at exactly the time they promise, but they do a reliable job at a fair price.
They are also open plenty of hours, so overall I'd give Nicholas's Fleet Street Shell and A+ for convenience.