I guess it depends on the character. My Death has a Scarecrow that is awesome ( he has spritely and may cast death blade, shield and trap), as well as a Fiendish Foo Dog that has spritely and a Shadow Phoenix that rocks. My Fire has a Phoenix that is pretty cool and a Fire Foo dog that isn't too bad either. And finally my Storm has a Kraken who is decent and a Lavender Foo Dog that shows promise.
For me, it's hard to decide. I like to match my outfits, pets, and mounts. My Darkwraith set (darkwraith outfit+gargoyle+brown warg) is usually my Gargoyle, Boomer. Sometime's I like to use my Dastardly Magician, even though he does practically nothing. My Balance set (sprocket's feline hat+karahahn parched tunic+proud lion), I usually use my Leopard, Daisy. She can give me some power ups and a smoke screen spell. In looks, the pet I own, a Dastardly Magician would have to take the prize. So elegant and classy! Competitively, I like my Gargoyle. Dark Pact, may cast berzerk, and empower. But I do love all my pets! They're awesome!