If you have intention of playing Russia or America and want to buy new crew slots, best way of leveling the crew is putting helicopter to those crew slots. Helicoper is like an afterthought to afterthought to the core game, therefor pilot crew skills doesn’t affect helicopter performance, except logistical tree which is shared by all vehicle types.
If you fancy Heli PVE/ground assault, you can take advantage of the fact and put chopper to level 1 crew, level them along with unlocking choppers. I suggest maximizing reload speed in the logistical services, then the rest 3: repair rank to fit your chopper then repair speed then weapon maintenance. In heli PVE, repair speed won’t matter much since you will probably die and restart like that, reload is done in the air. But this way, you can pad crew level and or just don’t spend crew talent points until you put tank or aircraft there.