Hard to answer question,
it depends largely on the class that you play which pet is best.
It also depends on what you want to do with the pet.
Want it to tank for you (Gnoll, Angel, Yeti, Hawk, Orc) , or do big damage (Kentauros, Cerberus, Red Pixie, Salamander) , or maybe you want it to heal you (Blue Pixie, Unicorn).
Each kind of pets has its pro's and it's cons, so it largely depends on how you play which pet is best.
Also, some pets are more easy to obtain then others.