Actually the T-64 was considered initially superior to the T-72:
From
http://www.fprado.com/armorsite/T-80U.htm
The
T-80 was a direct descendant of the earlier T-64, which was a high-technology main battle tank designed by the Tank Design Bureau of Kharkiv that went to revolutionize the Soviet tank concept. This new tank design concepts, were developed under the leadership of Aleksandr A. Morozov, and the T-64 became the first main battle tank of the new generation of Soviet-designed tanks. The T-64 was the first tank in the world to feature an automatic loader, enabling the crew to be cut from four to three - commander, gunner and driver.
The T-64 is thus considered to be a landmark in the history of tank development in the Soviet Union, as every tank of the Soviet T-series entering service ever since (specially the T-80, T-80U, and its posterior developments) was influenced by the concepts initially introduced by the T-64 tank design.
Unlike the T-72, the T-64 was only used by the Soviet Army and never exported. It was superior to the T-72 in most qualitative terms, until the introduction of the T-72B model in 1985. Both tanks armed elite and regular formations in Eastern Europe and elsewhere, the T-64A model being first deployed with East Germany's Group of Soviet Forces in Germany (GSFG) in 1976, and some time later in Hungary's Southern Group of Forces (SFG).
BTW, T-90 was developed from T-72