All you need to do is get a high lvl toon to invite you to the last boss in each act. Beat the boss then go straight to next act's last boss......and so on.

And yes, a quest can be done over and over and over. I grinded hell act 1 up to the wretched mother quest. Turned that quest in for 44k xp. Takes 5 min.
Quest xp plus xp from mobs kills totaled was getting me a full lvl at 50 in 40 mins. Mindless running and killing then reseting quest was faster lvling then actually playing through from quest to quest