The National Theatre at Francuska 3  ( or better known at as the theatre at the square named KOD KONJA ) has very good international productions of ballet or opera performances. Check with their office at narodno pozoriste

They repertoire for April included Don Quixote, La forza del Destino, Aida, Hamlet…You can get the first row in the stalls for mere  £ 10 BUT as Serbian people are very passionate  and they may make remarks during performance so get a box for the same amount. The best seats in the boxes  are A5 seats 1 and 2. Boxes usually have 4 seats and for a party of 4 they are excellent value!

The other place to check for good performances is the Sava Center at New Belgrade.