We had a very good car rental experience with Sixt in El Calafate airport in January 2018.
Before renting a car in El Calafate (for two days) we read this forum to have feedback from past experiences to find the best car rental company. And it wasn't very encouraging.
But everything went fine with Sixt.
We first easily and conveniently made contact through WhatsApp to know what type of car was really available. Then, on arrival, we were upgraded. We were provided a recent car in good condition (Renault Duster). At last, the agency was very accommodating for the car return.
It was a very easy and straightforward car rental experience at a price similar to other agencies.