My favourite car rental company is without a doubt Hertz Car Rental!

For 10% discount applied at checkout, click the link below:
https://www.hertz.is/asasteinars-hertz/

Hertz is one of few rental companies that have a counter inside the airport, so you'll be much faster at picking up your vehicle on arrival.

They also have an extensive coverage around Iceland and a very good customer service. Most (If not all) of the cars come with unlimited mileage and zero hidden cost.

The unlimited mileage is a huge plus in Iceland as it can always be so tempting to drive down just one more road!