I don't know if it's the fastest, but I always took my pets SS/LB farming. Double experience from the bounties, and it's in HM, and they don't take damage so you can even get Dire if you really want it.
It depends on what evolution you want for your pet. I prefer hearty because the pet is between me and the foe. Hearty has more HP so they stay up longer. I usually go to where the average foe is about 3-5 levels above that of my pet and let them fight with me keeping them as much alive as I can. If I wanted a dire (they do more damage) I would still do the same but with a beast master build and let the pet do all of the fighting. A pet can level when it is dead, but that is no way to get a dire or elder pet.