It depends. Are you training solely for the EXP or do you want some bonuses on the side? If you're in it for the EXP, calculate if the mobs you can 2-hit gives twice (or more) the EXP the mobs you can 1-hit. But, it's more complex than that since you have to consider the spawn rate, etc. If you're in it for the bonuses, I'd recommend locations that drop -- not only equips that you can NPC -- but equips that sell for a lot when you achieve perfect stats (Ulu Entrance / Ulu 2 / GS2 are the only locations that I know for that) The best solution is to really test it out for yourself. Train for 30 minutes to 1 hour at the location that you can 1-hit (i.e. GS2) and do the same in the location where you can 2-hit (i.e. Skele). See what works out best for you.
In terms of exp I'd say 1 hit skele>1 hit ulu 2>2 hit skele>the rest If you can't 1 hit either skele or ulu 2 you might want to try WS/ulu 1 or something though, because it gives more money than a map you 2 hit (you can sell leech+get items)
