Do you mean who was considered a worker in the USSR, or what constituted as the quintessential worker?

If you mean the former, then the USSR's consideration of the worker is more or less derived from the Marxist notion of the worker; that is, the proletariat and the working class. Taking a look at the Communist Manifesto, you will see that the worker is simply defined as the working class; the blue-collared manual labourer who used to work for the bourgeoisie, labourers who sold their labour to increase capital. Of course, to have a 'definition' of what a 'worker' actually is is quite problematic, for Marx's historical materialism delineates various nuances of the workers in differing epochs. But for the purposes of the USSR the general definition of a worker is the Marxist-Leninist conception.

With regards to the quintessential worker, I will say that the definition would be one that is outlined in the Stankhanovite movement.