Air serbia is really ok since the airline is managed by Ethiad and has a strong strategy partnership with them.

The short haul like BEG > OTP are done with old ATR 72 but except that the A319 / A320 are in very good conditions. Heard they will open BEG > JFK this summer with brand new A330 leased to Ethiad, can be an option for connecting in Europe.

Good if you have Ethiad or Alitalia status, brings you miles and benefits.

BEG airport is small and convenient (not a lot to do, but everything is smooth).

+ they propose a decent wifi onboard (not cheap but very convenient).