Material context matters, but anything that would help maintaining “soviet” organisation after the revolution. Truck drivers, bikers, hardware-capable technicians, but also nurses and medics, farmers, depending on the society maybe even programmers, and also military-capable workers.

It all depends on what sort of proletarian power structure you’re building and what are the key skills required to maintain and grow it both during repression and after victory. If there’s lots of rails, train engineers. If urban areas are sparse, maybe radio operators.