Tank pets take a lot of hits without flinching, while you lay on the damage.
The answer, as so many do, begins with another question: What kind of pet do you want? Hunter pets break down into three basic categories. Hopefully, it will finally be some kind of definitive response to the question, 'I've got this new spell, now what should I tame?' It's a culmination of data drawn from experience, the data collection of the Good Intentions Guild and Petopia, and the input of the many hunters on this forum.
We get this question at least once a day on the forums: 'What is the best pet?' The answer is complicated and it's silly to be saying 'it depends' all the time, and tiresome to continually point people at Petopia, so I thought I'd make this thread to lay the question to rest.