If you intend to use it for battle, then Breed is more important than rarity. You can use a Blue Stone to make a pet Rare, but you can never change its breed, and some breeds are just useless for battling in some species. For example, S/S Fiendish Imp is the only breed of Fiendish Imp worth getting.
Ummmmmmm … as a vague guideline, maybe, but It’s not that simple.
- Any given species may not have S/S and/or P/P breeds
- The most often best breed for a species depends on the moves chosen. Use different moves, you may want a different breed
- The best breed for a specific battle depends on the tactics of that battle. Usually, it’s good to be faster than your opponent, but in a few strategies you need to be slower because of timing
- In PvP, Speed tends to be worth more than it is in PvE
- More Power is always good. More Health is always good. With Speed you ideally want to be just 1 Speed faster than your opponent. Every unit of Speed above that is not only wasted, but costs you in Power and Health.
There are also a couple of interactions with racials that apply to specific families. For example, Speed is on average worth less to Undead and Mechanicals than any other family. For Flyers, Health is also Speed for half of their lives. For pets with strong heals or shields, Power is also Health. For Humanoids, Health increases their racial healing.
If I was making a generic list of breeds, I’d start it
S/S > P/P > H/H>=H/P
but that will be very wrong lots of times. For example, P/P >> SS for Shore Butterfly, Gleamhoof Fawn, Autumnal Sproutling and I’m sure several others. I tend to favour H/P for moths, but the Silky Moth (S/S), Yellow Moth (P/P), and Oasis Moth (H/B) are exceptions.
I did a survey of breeds preferred by top battlers/collectors here
That lists the distribution of breeds they chose for each species. I think it’s safe to say that the most common breed chosen is always worth getting, and if the second most common is very close to the first, they are both worth getting.
See also this:
and my comment about the most used breeds of each one below.