We were very impressed with the process of hiring a car with Sixt in Rovinj, Croatia. We booked a BMW X1 and were very surprised when we arrived and received a BMW X4. The car was very clean and all pre-existing damaged was documented to avoid any confusion on drop-off.
The process was very efficient and the staff were a pleasure to deal with. We also dropped in a different location and it was seamless.
One thing to be aware of is they do take a large deposit (€1800) from your credit card to cover insurance, but this is refunded a few days after you return the car safely.
Definitely recommend using Sixt for your car hire.