It is not a simple question.
Please read this thread, which has answers from several people, including my answer, to the same question.
Feel free to read it two or three times, looking up pets as you go. As I say, it is not a simple question.
I'll try to recap the process again. The more times you hear different presentations, the better!
Choosing the best breed(s) for a specific pet can involve many factors, and experienced battlers may still differ, but there are two major steps.
1. Do you want Speed?
2. How valuable is Power vs. Health?
in that order.
1. If you are using moves which require you to go first for their full effect, you should prioritise Speed. It's hard to find a reason to get anything but the fastest Rabbits with Dodge, Flurry, Burrow, or Scalded Basilisk Hatchling with Thrash and Crystal Prison, because if they are slower, those moves are not worth much.
1a. However, sometimes, if you already have the very fastest rabbit (357-speed) you may well want to get a second rabbit (341-speed or 325-speed) for cases where 357 isn't needed, and the extra bit of power or health you gain in the trade-off against less speed can be helpful.
In the same way, while you might prioritise the fastest generic Frog (Leopard Tree Frog S/S breed) you also might choose to get the most powerful Frog (Yellow Bullfrog P/P). When you know your Leopard Tree Frog will be faster than its opponent, you will likely use that; when your opponent is faster than your S/S Leopard Tree Frog, so its speed is no use, you will pull out your more powerful P/P Yellow Bullfrog.
1b. Also however, sometimes you may use different movesets on a pet, which will change the priority.
Sometimes a pet would be gr-r-reat! if only it had high speed. But no breed has a speed high enough to make its moves worth using. Such pets tend to be viewed as disappointments by many battlers.
2. Having decided the speed factor, you now choose between Health and Power options. Again, this may not be simple. Some racials play into this: Humanoids recover 4% of their Health when they do damage, which argues for preferring Health on Humanoids, and Flyers get a 150% speed buff when above half-health, which also argues for Health. If you intend to use a heal or shell mitigation on the pet, that is a strong argument for Power, since power increases the amount of the heal or protection.
For a cheat-sheet of answers, I would direct you to this survey of breeds chosen for pets:
So if you want to find which breed to prioritise for, say, Qiraji Guardling, you can see that S/S is most popular, (because when using Kick, you need to be faster than your opponent), and P/P is next most used.
As your roster grows, you may choose to keep two or three of the same species with different breeds for different conditions. Perhaps the simplest case to explain is Iron Starlette. The Starlette is usually used Wind-Up, Supercharge, Wind-Up. That does a ton of damage. It also has a speed increase, Powerball, which is most commonly taken. The P/P breed does a lot more damage in its Wind-Up than the H/H breed, but the H/H breed is more likely to survive for a second cycle. Sometimes you want to be sure to be faster before you unleash your big Wind-Up, to prevent the opponent Dodging it. The P/S breed needs one less Powerball to get to the same speed than the P/P or H/H breeds, and if that is the priority, then you would choose the P/S. I keep all three breeds, and use the one most appropriate for each case.
You need to be using the addon Battle Pet Breed ID which you can find on Curseforge.com. It will show you the breed of each pet without you having to work it out by memory or menthal arithmetic.
(You should also be using the addon Rematch - every battler should! - and probably Pet Tracker and maybe Derangement's Pet Battle Cooldowns, but those aren't about breeds, specifically.)
Also feel free to join the WarcraftPets Discord https://discord.gg/FBtGdKU
where people are delighted to opine on which pets in which breeds are better than other pets and breeds.
P.S. Apart fom the thread I already linked above, here are two recent threads with people discussing which breed would be best for newly-released pets. You may pick up more from practical discussions than from generalisations:
7.3 Breed Choices: Argus
BfA Wild Pet discussion