It's not so much where you level - in this game it's more what you're doing to level. In an effort to take the grind out of leveling it is almost impossible to level just by killing mobs. And since the majority of your XP is going to come from completing quests, your best bet is just to follow the money! Pick up a bunch of quests that are yellow or white to you (the sweet spot for a return of XP vs. energy expended) and complete them as quickly as you can. Don't kill much more than you have to, there is no percentage in slaughtering 15 pigs when the quest only requires 10...