Here is a quick way I have been using to level up. Only requires 1 game client and account so no need to switch back and forth.
1. select act 1 quest 1 part two: Rumford at the gate (hell)
2. complete first part of the quest
3. go north till you hit the checkpoint
4. complete the bonus part of the quest where you have to kill 3 wretched mothers (DON'T kill the wretched queen)
5. TP to town, start new game and repeat from 4.
Bonus: after killing the third wretched mother you can kill Sarkoth in the dank cellar (not for the loot but for the experience since he is worth about 4.8k xp)
How this works: You can repeat the bonus part of the quest indefinitely for 16.5k xp and 500gold + ~8k kill xp and + ~5k if the dank cellar is open. There are ALWAYS 3 wretched mothers on the way to the dank cellar!
normal run is ~25k xp (as shown in the video)
bonus run is ~30k xp
so average about 27.5k in lets say 40 seconds (can be done alot faster!)
That's 27.5k * 1.5 = 41.25k per minute *60 = ~2.5mil per hour (second run including Sarkoth in the video takes me about 30s which would be even more then 3.5mil/h!)
Note: this is for hell (lvl 50+), don't know if it is worth on lower difficulties!
Note2: Sometimes the quest does not reset after the first time you do the quest, just restart again and from then on it should work.