After seeing some mixed reviews, I've decided to contact Sixt Serbia and check their customer service.
Marija Crnogorčević was very helpful, polite and knowledgeable. She sent me an offer for 21 days rental of Renault Captur. This offer was slightly higer than what some other car rentals at Nikola Tesla were offering and much cheaper than Hertz for example.
The reason for this is those cheaper rentals are renting 10years old cars that are likely to break, compared to Sixt that has all the cars up to 2 years old and in a great condition.
Our car was actually brand new (only 50km on it). We were the first customers to rent it.
Besides Marija, other employees were also polite and helpful. We returned the car 2 days ago and our deposit is already back to our card.
If your flight is early in the morning, make sure to let Sixt know. They will work with you to accomodate your request. They open at 6am, but we were able to park the car at 5am, check our luggage in at the terminal and come back at 6am to return the key and Sixt employee checked the car quickly and did the paperwork. I received e-mail confirmation within 2-3 minutes.
I've seen some passengers waiting to return the car at some other rental agency that opens at 8am and their flight was at 6:20am. Not a good experience. Make sure to arrange drop off with the rental agency or just rent a car from Sixt next time. I can definitelly highly recommend them. At least their Belgrade/airport Nikola Tesla offices.
Ahh yes, their Renault Captur was fun to drive, a lot of equipment and even built in GPS, wireless android auto and apple car play and much much more.
Cars that you usualy rent in the United states are mostly basic with no equipement. This was a very nice surprise.