Gosh... I remember back before the pet families existed and you bought the skills you wanted your pet to know. I mostly used stamina/armor/resistance specc'ed kitties for leveling back then because they were my favorite. Then the first part of BC happened and pet aggro was so high, that I was co-tanking instances with a sporbat (which had no special ability back then). Suddenly gorillas showed up with thunderstomp, and everyone wanted one. I didn't really like them though.

I was happy when they introduced the pet families, but the news mentioned around Blizzcon that they were considering making the pet families more of a pet spec kind of excited me.
/OldTimerOff
All of that being said (<_<;;;), I like crabs and shale spiders if I need a really tanky tank pet. Shale spiders are extra nice for their buff aura, and since they are made of rock that just makes me feel better about them!

Crabs are great for their low profile, but before you can spec into the growl buff and thunderstomp a bear might be best.