Thief, and the reason you would never want to Power level it:
There is an important Dynamic that is learned whil leveling thief, especially in the early levels.
Kill more often for less reward.
If it takes you two minutes to kill monster X for 100 exp and you end with minimal HP, and you can make 25 EXP killing monster Y at the rate of 3 monsters every minute and come out with more HP to continue fighting... Fight the lower mobs.
I learned this rule the hard way. I was level 8 as a thief a record 16 times. Because I was trying to take on DC+ and EM mobs when I was first starting out, as a newb, with a dagger... I later realized that if I purged West Ronfaure of all it's rabbits, I could level once every hour up to level 7, and get to level 12 at about 2hr/level, restign very little, and amassing lots of items from creatures to fund my war machine. Later you poke Beastmen in Palborough Mines, Ghelbesha Outpost, or Giddeus, till you hit 15.
then you will be ready. And in the process learned 2 relatively important skills: EXP/hr > EXP/Kill & How to avoid aggro.