Key Warden Farming Route
Fields of Misery (at least two Nephalems, if you are lucky you can get five NV) -->Northern Highlands (To farm until you get five NV)--> Fields of Misery
If get get higher than four Nephalem Valor, you can go directly to the Misery. There you would run several herds of Elites and Champions before you run the Key wardens.
Power Leveling Route
Fields of Misery --> Highlands Crossing --> Northern Highlands --> Leoric’s Manor --> Halls of Agony Level 2 --> Halls of Agony Level 3 -->The Festering Woods --> Weeping Hollow
You do know that when you get five NV, your XP bonus would be increased enormously. So you should farm as long as you can instead of refresh your NV. Remember that, you are power leveling not key wardens farming only. We farm the Festering Woods last to ensure that you would not give up the previous task until you get the Key. I love the Highlands Crossing very much. When you cross the bridge, there would be a two layer cave. There you would definitely find one Goblin and One Resplendent Chest.
Fields of Misery is the best place in Act one to power level. There are two special dungeons. Each dungeon is filled with dense monster. Skip the Den cave which is a quest. Diablo 3 Gold farming can follow the power level route as well.
I personally farm the power leveling route till I get my Monk to the paragon level 100.