This one's easy: Elementals. They all look so cool, but they're such a pain in the ass to level up until you've already got other high-level pets for them to sponge XP off of. As much as I'd like to have leveled Deiter the Dark Phoenix and ERMAGERD PERBLE, it's not efficient to do so early on - although now that I can easily knock around some of the trainers in Pandaria, that may help a bit. (Although the fact you get hurt when you surrender is going to affect my ability to easily farm the Pandaren Earth Spirit trainer for XP. Oh well.)
There is one exception, who I recently leveled to 25 because he's just so useful: Grumpy Gustavo, my Terrible Turnip.