If you look on this page it'll show the pet names and damage shield damage. The Greater summoning fire has an inate damage shield of 28 which will tear mobs apart, especially if you can find some fast hitting mobs. Granted it has lousy mitigation and very few hitpoints but if you toss some summoned armor and a temperance onto the pet it'll be like a baby mage in god mode.
It's been a while so I don't know if the pre 50 pets were revamped, but back in the day the fire pet was the absolute beast (even prior to temp, toys). Like someone before said, even though it had the least HPs/mitigation etc... it's innate dmg shield, which at those levels are way better than what you can cast on him, just dropped mobs amazingly fast. Prior to lvl 50, it was my pet of choice. However, under certain situations you would use other pets, like air for a lot of caster mobs and earth for the really heavy hitting ones.
After 50, fire pets turned into pathetic wizards ... that is until the last fire pet revamp.
As far as the monster pets, they are mainly fluff. There was a time, IIRC MS3 or/and 4 had the best survival rating for raids because of the bull rush and proc, but that's a thing of the past.