For DE, Act2 Vota.
For xp and legs, act1, get 5 nv ASAP from festering woods, Leoric manor. Then go to Fields of misery, clear all white. Don't miss Decaying crypt and Scavenger cave.

DE = Demonic Essence. Reagent to craft the most powerful item in game (at the moment). Only drop from Elite. Act 2 Vault of the Assassin (VOTA) has highest packed Elite in one place.

5 NV = 5 Nephilim Valor. You gain 1 stack after 1 Elite kill. 5 is max. Will grant you 15% times XP per stack. With 5 stacks, you get 1.75 times more bonus XP. The idea is to get this 5 stacks as fast as possible, avoid white if you want. Then, go to Fields of Misery, currently the most packed large map in game. What is even better is that, there is 30% chance of Decaying Cave. This cave is XP gold mine.

Act 3 is still okay, but it's proven that Act 1 gain more XP per time...