Sure, here are some:

Georgi Agabekov, uncovered 400 KGB agents in Iran.

Vladimir Petrov, uncovered 600 KGB agents, including Kim Philby.

Gordievsky, head of KGB residenture in London, exposed all of it.

The failure of Berlin Tunnel.

Polyakov, head of GRU China sector. Exposed 1650 Soviet and 19 foreign GRU agents and operatives.

Penkovsky, provided 7500 pages of documentation on Soviet missile designs and rocket fuel types to CIA.

The KGB had a much harder time recruiting once the realities of life in the Soviet Union became widespread after WW2.

Operation RYAN nearly resulted in a 3rd world war in 1983 - purely based on KGB intelligence failures combined with a paranoid Soviet leadership:

https://en.wikipedia.org/wiki/RYAN