I did a quick search and didn't see this posted anywhere. It may be elite, or it could be that people don't care because there are better methods to power level, but regardless I think this deserves to be posted here if its something people aren't aware of.
You can enter the pony level without killing diablo (which any of us that played D2 would expect as a pre-req). Basically you can start a new game with a level one, and have a friend come in and open the pony level for you, and clear it. The experience isn't as amazing as you would first hope because of how low your level will be compared to the mobs but in testing I've found that from level 1-20 you will get anywhere from 1-1.5 levels per run, which if you're speed running can be done in 5min. So that works out to about an hour and a half for level 1-20.
You can then do the same with Nightmare and Hell testing with a friend we were able to do
Nightmare ponies level 30-35 a little less than one full level per run.
I haven't tested in hell yet so I'm not sure how good the experience will be for a level 50 +/- that has just gotten to hell mode.