There are two lounges at Belgrade airport (BEG), which, incidentally, is undergoing major renovations. Both accept Priority Pass. The Business Club lounge is the older of the two and has accepted Priority Pass as long as I can remember. The Air Serbia Premium Lounge is new to the PPass network--it joined maybe a year or two ago. Both are located in the A concourse a few meters away from each other. But there is a significant difference in feel and quality.
The Business Club lounge is usually crowded; seating is older leather chairs in close groupings. Food is adequate--salads, sandwich fixings like sliced meat and cheese, a hot dish or two, sometimes soup. Everything is presented buffet style. Drinks are decent (self-serve) and there is often a nice selection of Serbian bandy. Bathrooms are basic.
The Air Serbia Premium lounge is much lighter, brighter and cleaner (in design). There is a sit-down restaurant serving hot and cold made to order items. Maybe ten choices total (I had the roasted vegetable sandwich today with fries and it was excellent). Waiter service is the norm and everything comes with a smile. There is a bar area for spirits (probably not a significantly larger selection than the other lounge but certainly better presented). Bathrooms are new there is a shower (not sure if there are any conditions for use) .
Airlines use one or the other but as noted, PPass is accepted at both. So if that's what you're using to get in, I would definitely go for the Air Serbia lounge. At least until the airport completes its reconstruction (slated for Dec. 2023) and the renovated lounges open in the new wing of the airport.

Final note: the pictures of the two lounges on the PPass website kind of sum up the story.