As a night elf hunter, I preferred a cat as a pet. Their abilities are excellent for PVP and they attack extremely quickly. This makes it very hard for casters to use spells. In battlegrounds I used to send my pet after the main healer and just let it beat on the priest, keeping it from effectively healing the group. They had to either leave the pet to beat on him, or focus their attacks on it and not on another person. Cats are also excellent for ambushing or hiding and for chasing down enemies that run away.
Cat Stats:
Lower armor, Higher attack, Very high attack speed.
Stealth: A cat can disappear completely like a rogue. The first attack coming out of stealth will get a 35% bonus to damage. This is really useful for a night elf because it lets your pet disappear along with you.
Dash: This greatly increases your pet's run speed. Aside from being fun and saving time when you're sending it against enemies, it also is great for catching fleeing enemies and stopping people in PVP.
Diet: Cats eat meat and most importantly fish. If you have good fishing skill, you can catch food to keep your pet happy in just about any body of water you come across.
Of all the pets I came across in my travels, I decided to keep a blue frostsaber as my pet. It has beautiful hazy blue stripes and was once an elite enemy in winterspring. You can catch cats all over the world and they are all very good pets.