Many a thread here discusses that.
But breeds really matters on a few things.
Are you going to pvp? If not breed doesn't matter
What moves are you using from a pet? If it is one that hits more than once if goes first, stuns if first, a dodge or a blind then you want the fastest breed one can get of the type.
While on the flip side of that some moves benefit from being slower like dragon tail or undead pets.
What breeds the pet comes in. Some pets do not have much if any option on the breed others have multiple.
Lastly personal preference some swear by speed, others by power and still others like health.
Speed is very nice but most speed pets hit like wet noodles.
Power is nice and can lead to some awesome crits. Also, a lot if heals get extra heals based off of power.
Health makes for some nice tanky pets and aoe heals are based off of health so extra heals for health pets.
Basically it comes down to how you plan to use your pet. Then try to find the best breed that meets this.
Sometimes one doesn't have a choice though.