Тhere are several skyscrapers. Top 3:
1. Ušće Tower, ex CK (Central Committee of the League of Communists), 115m + antenna = 141m, built in 1961, damaged in 1999, reconstructed in 2005.
2. Genex Tower, also known as Western City Gate, 115 m (roof), built in 1980.
3. Beograđanka, 101m (roof), 1969-1974
And, of course, the first skyscraper on the Balkans, Palace Albania, 58m, finished in 1940.
There are also several other tall buildings, such as Eastern City Gate, or Banjica's Five Idiots (in Belgrade skyscrapers are usually called "idiots"), but I don't know their height.
Eastern City Gate