It depends on what you're planning to do with them.
For PVP, it largely comes down to speed and luck - if you end up against a team that counters your own, you can end up smashed before you get rolling. Like DJ said, there are some very popular builds out there - reflect/stun builds seem to be very popular in PVP right now (Flayer / Lil' Deathwing / Mana Wyrmling), for example. Countering those can be difficult, especially if they have fast backups.
For PVE, it's all about counters. What does the tamer have, and what do you have that works well against that? That having been said, a handful of pets are so grossly overpowered (pre-nerf Fluxfire Feline, for example) that they can be useful in almost any battle as a filler or backup.
If you're just looking for the "Petses with the Mostest," the Ranking List
should be of some interest to you.