Several airlines offer budget-friendly flights to Serbia. Some of the popular airlines known for their competitive fares to Serbia include:

1. Ryanair: Ryanair is a low-cost airline that operates flights to Serbia from various European destinations.
2. Wizz Air: Wizz Air is another low-cost airline that offers flights to Serbia from several European cities.
3. Air Serbia: Air Serbia is the national airline of Serbia and often offers affordable fares, especially for flights to and from neighboring countries in the Balkan region.
4. EasyJet: EasyJet is a budget airline that operates flights to Serbia from select European cities.
5. Eurowings: Eurowings is a German low-cost carrier that provides flights to Serbia from various European destinations.

It’s worth noting that flight prices can vary depending on factors such as the time of booking, travel dates, and departure airport. It’s recommended to compare fares and check for any promotions or deals offered by these airlines or other carriers to find the most budget-friendly option for your specific travel needs.