The carrion bird is currently the best pet for tanking. Its armor bonus coupled with screech gives it the most survivability. You can have it attack one mob with several others hitting it and still be able to keep mend pet up at all times without pulling threat.
Turtles are a useless pet. Shellshield sounds nice but the Screech beduff will, overall, cause your pet to take less damage, while not reducing damage dealt.
I use a carrion bird for farming and dailys, I use a ravager for instances and raids. Get a bird, you will not regret it.
As of the latest patch http://petopia.brashendeavors.net/html/mobs/mob_direraven.php are now tameable. You can get one at level 68. I have had mine for 2 days now. He is level 70 with maxed loyalty, it doesnt take long.