If you're just looking at leveling pets, try to find 3 pets that will be good against critters, beasts, flyings and/or aquatics, since these are the easiest to find in the wild.
Pets that can heal themselves will help with that as well.
I'm not sure what you have in your collection though...
Humanoids are generally a good choice, their passive is quite handy.
Mechanicals in general too, especially those with Repair, cheaper on bandages and they are good against beasts.
If you're close to 150 pets, I'd recommend the Celestial Dragon, he's very easy to level as he starts up with a good healing spell, a good damage spell that enhances healing and does extra damage to flying, and another decent damaging spell.
Fluxfire Feline will do awesome damage, if you can get your hands on one (they're by the Gnome starting area, so you can create a gnome on a new server until you find one). He'll take down any critter/beast in 2-3 rounds, making fights lots faster.
Magical Crawdad would be an awesome addition too, although he only truly reaches his potential at higher levels.