I have spent an awful amount of time in Thailand in the last 15 years and I have always rented cars. From all kinds of companies. Initially always from local companies as they had much better prices. Now there are much more options. What all of them had in common, however, that they tried to rip me off with "damages" that I actually did not make.
For the first time now I rented a car from Sixt. The price was rather competitive. The location at the airport is not ideal for people arriving from outside the country as the office is at the very other end of the terminal. A hassle if you come with a lot of luggage. But the service was friendly and they did not try to peddle additional insurance and everything was clear and direct.
I had to make a deposit of 8,000 THB for damages - which was normal and acceptable.
Sadly, I managed to put some scratches on the car in the narrow alleys of the old town. My fault! When I returned the car they produced their list for the cost of repairing the damages (in my case the front spoiler, and the two left doors that needed to be repainted). Their list said they had to charge me 6,000 THB for all damages. Very clear and precise. And acceptable as it was my own fault - and fairly priced. No fuss. All the others would have just kept my 8,000 THB deposit.
This will make me come back to them on my next visit. They have earned my trust!