I will try to arrange them by their usefulness in PvP. I tried to use the best possible ability setup that I could think of, and compare it with the breed id to see how useful they are compare to other pets with similar abilities.
1. Earth (breed #7 aka H/P)
Why? Fairly unique weather effect(only shared with Sumprush Rodent) that traps for 3 rounds every time a pet is swapped in. 25% chance of stun on damage ability. And a nice 1 round nuke with Stone Rush.
2. Air (breed #3 aka B/B)
Why? The crappiest breed id for overall stat gain, but as far as I've seen the only pet with the possibility of having AoE abilities in every slot. Situational, but its uniqueness gives this an edge over Fire and Water.
3. Water (breed #9 aka H/S)
Why not? The breed id makes the heal crappy. However, with round cooldowns counting while pet is inactive now, Dive and Healing Wave could potentially make it really hard to kill(but the crappy power will most likely put a stop to that), if you just swap it out when they are on CD. Tidal Wave for AoE or incase you meet those pesky gnome turrets and/or other objects.
4. Fire (breed #8 aka P/S)
Why not? The breed id gives it low HP and the speed is not good enough to matter, so basicly wasted stats. Immolate(burning) + Conflagrate are always a winner, but there are other pets with the same combo with better power. Magma Wave for AoE / objects, but again the power is really low so the only good thing is removing objects part.