Consider a large ATC facility, where a range of ATC services are provided and each position requires a dedicated person to manage (unlike small airports, where the same guy is ground + tower). Which positions are considered the most demanding, and it would make sense to assign a senior and experienced controller there?
I would imagine, Clearance Delivery is probably the easiest one, then Enroute, Approach / Departure, Ground and finally Tower. What's the training sequence recommended by the authorities for a new controller? After all, assigning a fresh trainee to LAX tower or JFK ground does not sound like a good idea.