Recently, the popularity of leveling through pet battles has increased quite a bit. I personally prefer questing, but pet-battles do have a few good points. Especially if you enjoy concentrating on something else while you're leveling.
Most places where people level through pet battles are spots with only 1 family type pet, so you can be sure to always have a pet that's strong against the pets you meet. However, no matter how easily you can kill another pet, you still have to kill 3 pets to get full experience, right?
Wrong. There is a spot in Deadwind Pass right next to Karazhan where the lvl 16-18 battle pet Arcane Eye can be found. When engaging this pet in a pet battle, it won't bring any companions along which means that you're guaranteed to fight these pets one at a time. Unlike other pets, however, you get full experience for beating this one pet. That's right - By beating just 1 Arcane Eye, you'll get the same amount of exp as beating 3 lvl 25 pets.
Pros:
You only need to kill 1 pet to get almost the same amount of exp as a quest on your level. It's potentially 3 times faster than most other pet-battle leveling spots.
The pets are all magic type, so as long as you bring a pet with a dragon type attack, your fights will be very easy.
The pets are all gathered within a small area, so there's hardly any travel time between the pets.
As of right now, there's hardly any competition. I still haven't met anyone else farming here.
Cons:
The amount of pets is limited and the respawn timer limits your xp/hour. This forces you to do either dungeons, bg's or stuff on other chars while waiting for the eyes to respawn.
From what I've seen, the pets can't be found every hour of the day. I still haven't found when it is that they disappear, but sometimes the area will just be empty.
A big part of the pets are surrounded by lvl 70 mobs. This limits your speed below if you're below lvl 70.
Arcane Eyes are magic pets, so you'll need at least 3 hits to bring them down no matter how strong your attacks are against them.
All in all, this is a very effective method. What I did was to catch a couple of Spawn of Onyxias in Dustwallow Marsh and a few Infinite Whelplings from Tanaris. All of them uncommon or rare quality. These pets are a couple of levels below the Arcane Eyes, but since they're dragonkin, it should still be easy to beat the Arcane Eyes with them. The speed will increase once your dragons get to lvl 16-17.