I propose Iskatu boss runs (Act 4 Quest 1).
In nightmare i can do one run in about 40-45 seconds from login to logout, which comes out to just over 2.2 million xp per hour. (of course you can do them on normal till you can beat nightmare, i wouldnt do it on hell though)
For that to work take the runes tailwind (for running to the portal) and overawe (for fast killing) and the passive fleet footed as well as a pair of shoes with +runspeed (obvious). Also a highdamage follower like the enchantress is helpful.
Also don't forget to skip the tyrael dialogue with the "close all windows" option (under key bindings) and buy a new weapon from the AH all 5 levels or so.
That is considerably faster than all other methods (kulle runs posted on the mainpage for example...)