It really depends on the character I'm playing and what I'm doing. If I'm farming, I don't like the flying pets because they always seem to land right on front of the ore or herb I'm after. If I'm running dungeons or lfr, I'll try to find a pet that matches my hunter pet when I'm on my hunter, or maybe an elemental if I'm on my mage or shaman. Anything goes on my priest. Something that just looks "right" to me that day.
![Very Happy :D](./images/smilies/icon_e_biggrin.gif)