I've been a customer of Sixt for years. They're the number one in Germany for a reason. The service is phenomenal, the personnel is always trying to offer the best service to their customers, and the cars are usually very new, well serviced and high trimmed.
I rented GFAR and they had a massive selection of new, high trim cars. Nariah, the representative, offered to let take a look at the cars before I decide which one I want to take and even unlocked it for me to sit inside. She offered but didn't pressure (!) me to buy any additional insurances, which isn't the case with many other rental companies.
The 2020 GMC Yukon XL I chose was immaculate with 0 damages, 10,000 miles, SLT (second highest) trim and additional options. Giving it a sticker price of almost $70,000.
The return of the car was as easy as can be. Get out, sign ,and hop on the shuttle that was waiting for me.
I can confidently give Sixt IAD 5/5 stars. The only criticism I have is that the station is so far out of DC. I hope Sixt opens another branch in the city.