This is 50% my own discoveries and 50% compilation of other people's "guides".
The fastest way I've found to level so far, while getting decent items/gold is to do this loop on Normal till you can breeze through NM, and then do it again on NM till you cna breeze through Hell.
- Start at Act III, 7. Kill Azmodan.
- Run by all the monsters unless you encounter elite mobs.
- Nuke down Azmodan, then immediately bypass the cutscene with space or escape.
- Grab his soul and your loot, then teleport back to the stronghold.talk to the guard spamming though his dialogue with space or esc then run to the armory, then the watchtower.
- Skip through the cut scene, then run through the portal.
- Run up to Tyrael, spam through his dialogue then run through the portal, spam through dialogue and kill Iskatu.
Rince and repeat.
This will net you a healthy amount of XP, gold, items, and is higher XP/h than just Azmodan or Isatu runs.
Edit: Here is a video