NEWS FLASH
Turkey's Atlasglobal will introduce additional flights from Istanbul's main airport to Belgrade from April 1, 2019. The airline plans to maintain daily services between the two cities, up from four weekly. As a result, there will be a total of 25 weekly flights between Istanbul and the Serbian capital, four of which will operate from Sabiha Gokcen Airport. Both Turkish Airlines and Atlasglobal are set to move from Ataturk Airport to Istanbul's New Airport at the end of March. Atlasglobal codeshares on a number of Air Serbia flights and its business class passengers have access to the Air Serbia Premium lounge in Belgrade.

Comments
I'd dare say it's one of the busiest routes in the Balkans.
https://www.exyuaviation.com/2017/12/atlasglobal-announces-sarajevo-pullout.html
Now they just fly charters there.
BEG-TGD 293.913
ZAG-DBV 268.173
BEG-IST 249.589
BEG-TIV 216.553
Numbers are for IST alone, SAW isn't included. As for ZAG-DBV from what I remember it's not doing as great in 2018.
Seems like BEG-ZRH is the busiest route in ex-YU:
1. BEG-ZRH 368.815
2. ZAG-FRA 321.907
3. ZAG-MUC 191.990
In Q1 and Q2 BEG-ZRH grew by some 10.000 passengers so next year it might have around 400.000 passengers which is impressive.
Busiest routes out of Croatia in 2017 were dominated by ZAG.
1. ZAG-FRA
2. ZAG-DBV
3. DBV-LGW (239.024)
4. ZAG-SPU
5. ZAG-MUC
The next non-ZAG route is SPU-LGW (153.484) on the 9th spot.