My favourite car rental company is without a doubt Blue Car Rental!
For 5% discount applied at checkout, click the link below: https://www.bluecarrental.is/?affiliate=asasteinars
I know they have extensive coverage around Iceland and have 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!
open in google