Thief

Pick 50 locks and 50 pockets (both counts). Locks are everywhere - practise on every chest and door you find. Pickpocketing 50 needs the sneak-and-steal on NPCs; the Thieves Guild questline throws plenty your way, and you can pickpocket the same sleeping NPC repeatedly (take gold, they don't notice) to rack it up fast. Both counters run independently.