The world's largest spy agency is

Some experts consider that the largest intelligence agency around the world is ISI.

Explanation:

The ISI, Inter-Services Intelligence is the main intelligence agency in Pakistan, it is in charge of the analysis of all the information related to national security around the world. Its main task is to work with the Pakistan government to provide intelligence.

The ISI uses military officers but it also enlists civilians. This organization achieved recognition around the world because of its participation in the Soviet-Afghan War in which the ISI worked in collaboration with the Central Intelligence Agency (CIA).