A fast and uncrowded gas station especially if you have the OB/Olis token or discount card which by the way you can get it almost everywhere at Olis Stations. They have free coffee or discounted depending what sort of discount card you have and/or token. Easy to access 24 hours!
Open 24/7 and within a short distance of Keflavik airport - very handy to fill up rental cars before returning (less than 10km from FairCar Rental). Decent prices for Iceland and large car park, very near to two large supermarkets too.
Very helpful gas station attendant helped us find our way around town. Provided some helpful hinters for locations to visit. Station was clean and well ordered as was the bathroom.