Back to the roots, basically.
We either need a second account or someone willing to help us, because we're getting back into Dungeons for leveling boosting.
But the thing why it's so good now is due to the way the level scaling works.
You can expect about 60%(a single rare in ZF added +20%, excluded from the average run) xp PER RUN. Completely insane.
Let's for example use the toon I am currently boosting. I went with my boosting character (Level 100 Monk) in and forced the scaling of all of the ZF mobs to their max for the dungeon, which is 60 in the case for classic dungeons.
This either intentionally(or unintentionally) yields in drastic XP increase for lower level characters.
Now you might be familiar with how incredible it works with ZF specifically, but this isn't where it ends. You can practically apply this line of boosting to any dungeons, as long as you can efficiently pull (almost) the entirety of dungeon and bring them back to where your toon is waiting (preferably the entrance to save time).
I boosted my Monk all the way from 39 to 58 and then went into Hellfire Ramparts for about ~6 levels and then decided to give Mana Tombs a try, which I prefer due to the higher mob density.