Yeah basically what Nagini said.
It really depends on what you're going to do with each pet.
But overall, yes, breed does matter. With the Val'kyr, if you only intend to use it for "Unholy Ascension" then it will provide the same damage for your team no matter which breed you use. But if you're using it for any of it's personal hitting abilities, the B/B has both better damage and speed.
For any rabbit you basically have two choices in attack style. A fast approach where you get three hits off "Flurry", or a slower attack with more power using "Scratch." If you want to hit with scratch, and maybe Adrenaline Rush, the H/P Highland's Cottontail is the strongest. Otherwise there are several fast rabbits to chose from. The Highlands in a S/S is a good compromise because it has a higher damage to speed ratio; but that's just my choice—I also have faster rabbits should I feel I need one.
If you want to use a pet with "Explode", the amount of damage you do is based on a percentage of your overall health-pool (40%). So the higher the health, the more damage you will do. That makes the H/H Blackfuse Bombling the must have for explode. But either a Netherspace or Minfernal in H/H is second best. Otherwise neither Netherspace or Minfernal have abilities that benefit from speed. So then I would try to get a P/P Netherspace which will do damage according to it's 333 power (provided your Meteor Strike hits and crits).
And some pets have such low speed that chances are you will probably always be going against something faster no matter which breed you chose. In those cases I will take the one with the greater health (often a H/P or B/B), which can help you take a couple more hits from the opponent (in some cases this completely offsets the opponent's speed).
Here, maybe this will help a little in deciding which ones to capture or use a stone on. It's a little rough, and doesn't take into consideration all abilities. But it can be a basic guide to which are most powerful, fast, or durable. (Sorry for the notations "NEED" at the ends of each some, that's just how I keep track of what I still need. And I'm always adding to this, so anyone can let me know if I've made errors, oversights, or need to add something.)
BEST IN SPECIES
(NOTE: This is a general P/S/H comparison chart, and does not take into consideration all abilities.
Also, it only covers extremes in either stat. There are many balanced (middle of the road) breeds as well).
BEETLES:
POWERFUL!
1319/341/260 (P/P) Lava Beetle (Having high hit, and "Cauterize, makes this the most powerful and durable!)
1337/341/276 (P/P) Twilight Beetle
1465/305/260 (H/P) Fire Beetle (Has "Cauterize")
FAST!
1400/276/309 (S/S) Gold Beetle
DURABLE!
1644/276/260 (H/H) Savory Beetle
CATS:
STRONG!
1237/341/276 (P/P) Cornish Rex Cat
1302/322/276 (H/P) Black Tabby Cat
1237/305/305 (P/S) Cat, Bombay Cat
1221/233/289 (P/B) Feline Familiar
FAST!
1337/276/341 (S/S) Cat, Siamese Cat
(NOTES: The ordinary Cat and Bombay Cat have the best speed/power ratio. Feline Familiar has unique abilities in the cat family.)
CHIPMUNKS & SQUIRRELS:
STRONG!
1465/289/273 (P/B) Alpine Chipmunk, Grizzly Squirrel, Squirrel
1546/289/260 (H/P) Alpine Chipmunk
FAST!
1400/260/325 (S/S) Alpine Chipmunk, Red Tailed Chipmunk, Grizzly Squirrel, Squirrel
1465/273/289 (S/B) Red-Tailed Chipmunk, Grizzly Squirrel
1400/289/289 (P/S) Nuts
CRABS:
STRONG!
1481/357/211 (P/P) Emperor Crab
1319/357/244 (P/P) Spineclaw Crab (Faster than Emperor; but has lower healing.)
1481/325/244 (P/P) Spirebound Crab
(NOTE: Lava Crab is unique in that it has "Cauterize" and can hit back line pets.)
HEALTH!
1806/292/211 (H/H) Emperor Crab
1806/260/244 (H/H) Spiritbound Crab, Strand Crab
(NOTE: "Spirit Crab" is not listed because it is "Undead". Though it is a must in any arsenal)
FROGS:
STRONG!
1562/309/244 (P/P) Yellow-Bellied Bullfrog
1709/273/244 (H/P) Huge Toad
1562/273/273 (P/S) Biletoad
(NOTE: Although the "Swamp Croaker" has middle of the road stats,
it has a unique advantage with "Croak", and "Bubble".)
FAST!
1481/244/325 (S/S) Leopard Tree Frog
1546/257/289 (S/B) Small Frog
1627/255/289 (H/S) Mojo
HEDGEHOGS & PORCUPINES:
STRONG!
1587/289/252 (H/P) Vengeful Porcupine
1481/276/276 (B/B) Silent Hedgehog, Clouded Hedgehog
(NOTE: Harmonious Porcupine has good self healing, and group heal.)
FAST!
1400/260/325 (S/S) Clouded Hedgehog
(NOTE: Spectral Porcupine is "Magic", and not Beast.)
LIZARDS:
POWER!
1400/341/244 (P/P) Diemetradon Hatchling
1384/305/276 (H/P) Plains Monitor
1465/289/276 (H/P) Spiky Lizard, Horned Lizard, Twilight Iguana, Warpstalker Hatchling
(NOTE: Scalded Basilisk Hatchling has lower baseline power, but has a 25% power buff.)
FAST!
1400/341/244 (P/P) Spiky Lizard, Lizard Hatchling, Twilight Iguana
MAGGOTS:
STRONG!
1709/289/227 (H/P) Mr. Grubbs
DURABLE!
1887/260/227 (H/H) Devouring Maggot, Festering Maggot, Maggot
MOTHS:
STRONG!
1390/341/260 (P/P) Yellow Moth
1400/289/289 (P/S) Ashwing Moth (also has a 9 round hit chance buff.)
FAST!
1319/255/357 (S/S) Silky Moth
1481/227/341 (S/S) Fungal Moth
1500/260/325 (S/S) Grey Moth
DURABLE!
1644/244/292 (H/H) White Moth
RATS & MICE:
STRONG!
1400/341/244 (P/P) Giant Sewer Rat
1400/289/289 (P/S) Prairie Mouse
1400/273/305 (P/S) Mouse
1400/273/289 (P/B) Highlands Mouse, Stowaway Rat, Yakrat
FAST!
1400/244/341 (S/S) Fjord Rat, Grotto Vole, Highlands Mouse, Mouse, Rat,
Redridge Rat, Stormwind Rat, Stowaway Rat, Undercity Rat
Wharf Rat
1465/257/305 (S/B) Black Rat, Carrion Rat, Fjord Rat, Undercity Rat
RABBITS:
STRONG!
1546/289/260 (H/P) Grasslands Cottontail
1546/257/292 (H/P) Rabit
FAST!
1400/227/357 (S/S) Arctic Hare, Mountain Cottontail, Rabbit, Brown Rabbit, Snowshoe Rabbit, Spring Rabbit
1400/260/325 (S/S) Grasslands Cottontail
(NOTE: "Darkmoon Rabbit, with S/S and balanced breeds, is in it's own class, as it has "Huge, Sharp Teeth!")
ROACH:
STRONG!
1627/257/276 (H/P) Deepholm Cockroach (have), Fire-Proof Roach, Nether Roach,
Resilient Roach, Roach, Tainted Cockroach
1562/244/292 (B/B) Undercity Cockroach
1709/240/276 (H/P) Irradiated Roach
1546/240/305 (S/B) Cockroach
FAST!
1841/227/341 (S/S) Cockroach, Deepholm Cockroach, Resilient Roach, Undercity Cockroach
DURABLE!
1817/211/276 (H/H) Irradiated Roach,
1806/227/276 (H/H) Cockroach, Deepholm Cockroach, Resilient Roach, Tainted Roach, Undercity Cockroach
SCORPIONS:
STRONG!
1400/289/289 (P/S) Durrotar Scorpion, Leopard Scorpian, Stripe Tailed Scorpion, Scorpid
FAST!
1400/260/325 (S/S) Leopard Scorpid, Scorpling
SNAILS:
STRONG!
1790/305/195 (H/P) Shimmershell Snail
DURABLE!
1969/276/196 (H/H) Shimmershell Snail @ Darkshore
SNAKES:
STRONG!
1319/325/276 (P/P) Death Adder (need), Tree Python
1319/305/289 (P/S) Moccasin
1465/289/276 (H/P) Emerald Boa
1465/289/273 (H/B) Crimson Snake
1384/289/289 (P/B) Adder
1319/289/305 (P/S) Cobra Hatchling, Ash Viper, Rattlesnake, Rat Snake
FAST!
1319/260/341 (S/S) Death Adder (need), Grove Viper, Moccasin, Rock Viper, Sidewinder, Snake, Temple Snake
1384/257/322 (S/B) Albino Snake
DURABLE!
1644/260/276 (H/H) Elder Python
SPIDER:
1237/341/276 (P/P) Twilight Spider, Widow Spiderling, Feverbite Hatchling
1384/305/276 (H/P) Amethyst Spiderling (NEED)
1237/305/305 (P/S) Dusk Spiderling (NEED), Desert Spider, Forest Spiderling
1319/292/292 (B/B) Crystal Spider, Spider
STRIDERS / SKIMMERS:
STRONG!
1400/273/305 (P/S) Eternal Strider
1481/260/292 (B/B) Dancing Water Skimmer
1400/260/309 (B/B) Mirror Strider
DURABLE!
1465/257/305 (H/B) Mirror Strider
FAST!
1319/227/374 (S/S) Aqua Strider
1465/244/322 (H/S) Mirror Strider
TURTLES:
STRONG!
1725/260/244 (B/B) Emerald Turtle
1644/260/260 (B/B) Turquoise Turtle @ Azshara
DURABLE!
1969/244/227 (H/H) Emerald Turtle @ The Jade Forest
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
STRONGEST IN FAMILY
(The "tanking" list is based on survivability mechanics, and not health pool alone.)
Aquatic
POWER!
1481/357/211 (P/P) Emperor Crab
1481/325/244 (P/P) Spiritbound Crab
1481/325/244 (P/P) Shore Crawler (Goblin and AH)
1400/325/260 (P/P) Purple Puffer
1562/309/244 (P/P) Yellow-Bellied Bullfrog
1400/305/273 (P/S) Toothy
1546/305/244 (H/P) Snarly
TANKING!
Magical Crawdad
Emperor Crab (either breed)
Emerald Turtle (either breed)
Swamp Croaker (either breed)
Spineclaw Crab
Beast
POWER!
1481/357/211 (P/P) Stunted Shardhorn
1400/341/244 (P/P) Diemetradon Hatchling
1237/341/276 (P/P) Cornish Rex Cat
1237/341/276 (P/P) Zandalari Kneebiter
1237/341/276 (P/P) Twilight Spider
1237/341/276 (P/P) Zandalari Ankelbiter (NEED)
1237/341/276 (P/P) Zandalari Footslasher (NEED)
1237/341/276 (P/P) Zandalari Toenibbler (NEED)
1441/325/252 (P/P) Stunted Direhorn (NEED - from achievement)
1400/325/260 (P/P) Baby Ape
1400/325/260 (P/P) Darkshore Cub
1400/325/260 (P/P) Snow Cub (NEED)
1400/325/260 (P/P) Fjord Worg Pup (NEED)
1400/325/260 (P/P) Razashi Hatchling (NEED)
1302/322/276 (H/P) Black Tabby Cat (NEED)
1221/322/289 (P/B) Feline Familiar (NEED)
1237/305/305 (P/S) Cat
1302/305/289 (P/B) Gundrak Hatchling (NEED)
1237/305/305 (P/S) Molten Hatchling
1237/305/305 (P/S) Ravenger Hatchling
1237/305/305 (P/S) Desert Spider (NEED)
1237/305/305 (P/S) Dusk Spiderling (NEED)
TANKING!
1441/325/252 (P/P) Stunted Direhorn
1237/341/276 (P/P) Twilight Spider or Widow Spiderling, both P/P
Cats: Cornish Rex Cat (P/P) Cat (H/P), or Feline Familiar (P/B)
Critter
POWER!
1197/370/249 (P/S) Red Criket
1319/341/260 (P/P) Scarab Hatchling (NEED)
1237/341/226 (P/P) Twilight Beetle
1319/341/260 (P/P) Lava Beetle (NEED)
1400/325/260 (P/P) Stinker
1319/322/273 (P/S) Perky Pug
TANKING!
1546/289/260 (H/P) Grasslands Cottontail
1969/276/195 (HH) Scooter the Snail
1969/276/195 (H/H, or H/P) Shimmershell Snail
Fast!
1400/227/357 (S/S) Spring Rabbit
1400/227/357 (S/S) Snowshoe Rabbit
1400/227/357 (S/S) Snowshoe Hare
1400/244/341 (S/S) Malayan Quillrat Pup (NEED)
Dragonkin
POWER!
1481/357/211 (P/P) Lil' Deathwing
1481/341/227 (P/P) Thundering Serpent Hatchling (raid perk)
1400/341/244 (P/P) Nexus Whelpling
1400/341/244 (P/P) Emerald Proto-Whelp
1400/341/244 (P/P) Spawn of Onyxia
1400/341/244 (P/P) Infinite Whelpling
1319/341/260 (P/P) Nether Faerie Dragon
1481/322/240 (P/S) Dark Whelpling (drop)
1237/322/289 (P/S) Crimson Whelpling (NEED)
TANK!
1400/341/244 (P/P) Emerald Proto-Whelp
1644/276/260 (H/H) Chrominius
Elemental
POWER!
1400/374/211 (P/P) Lil' Ragnaros (NEED)
("Fel Flame" is weaker, but has more reliable hit chances.)
1319/357/244 (P/P) Crimson Geode
1481/325/244 (P/P) Emerald Shale Hatchling
1400/325/260 (P/P) Amethyst Shale Hatchling
1627/322/211 (H/P) Core Hound Pup
TANK!
1481/325/244 (P/P) Emerald Shale Hatchling
1400/325/260 (P/P) Amethyst Shale Hatchling
Ruby Sapling (either breed)
Flying
POWER!
1400/341/244 (P/P) Yellow Moth
1319/341/260 (P/P) Imperial Eagle Chick
1319/341/260 (P/P) Nether Ray Fry
1319/341/260 (P/P) Wildhammer Gryphon Hatchling (NEED)
1481/325/244 (P/P) Dragonbone Hatchling
1319/325/276 (P/P) Bat
1384/333/373 (P/B) Brilliant Kaliri
1302/322/273 (P/B) Fledgling Nether Ray
1156/322/305 (P/S) Perrodax Hatchling
TANK!
1156/322/305 (P/S) Perrodax Hatchling
1400/341/244 (P/P) Yellow Moth
Humanoid
POWER!
1400/341/244 (P/P) Kun-Lai Runt
1319/341/260 (P/P) Lil' Bad Wolf
1400/325/260 (P/P) Flayer Youngling
1400/325/260 (P/P) Curious Wolver Pup
1400/325/260 (P/P) Sporeling Sprout
1400/325/260 (P/P) Qiraji Guardling
1400/325/250 (P/P) Harbinger of Flame
TANK!
Anubisath Idol
Flayer Younfling (S/S, P/P, P/S, or H/P)
Magic
POWER!
1156/357/276 (P/P) Mini Diablo (lol)
1319/341/260 (P/P) Twilight Fiendling
1400/333/252 (P/P) Netherspace Abyssal
1481/325/244 (P/P) Jade Oozeling
1400/325/260 (P/P) Minfernal (the H/H does better damage with "Explode, as explode works off a
percentage of health. Blackfuse Bombling is even better.)
TANK!
1481/325/244 (P/P) Jade Oozeling
Mechanical
STRENGTH!
1441/341/236 (P/S) Pocket Reaver
1400/325/260 (P/P) Son of Animus (NEED)
1400/325/250 (P/P) Sunreaver Micro Sentry
1546/322/227 (H/P) Lil' XT (NEED)
1237/322/289 (P/S) Fluxfire Feline
1481/309/260 (P/P) Tiny Harvester
1481/309/260 (P/P) Rabid Nut Varmint 2000 (NEED)
1481/309/260 (P/P) Personal World Destroyer
TANK!
1546/273/273 (H/B) Darkmoon Zepplelin
1237/322/289 (P/S) Fluxfire Feline
1465/289/273 (P/B) Rascal-Bot
1546/257/289 (S/B) Pierre
Annodized Robo Cub (either breed)
1400/325/260 (P/P) Son of Animus
Undead
STRENGTH!
1481/309/260 (P/P) Sen'jin Fetish
1627/305/277 (H/P) Infected Squirrel
1627/305/227 (H/P) Ghostly Skull
1627/305/227 (H/P) Lil' K.T.
1546/305/244 (H/P) Blighthawk (NEED)
1481/305/257 (P/S) Fossilized Hatchling
1481/305/257 (P/S) Scourged Whelpling
1481/305/257 (P/S) Crawling Claw (NEED)
TANK!
1881/260/227 (H/H) Infested Bear Cub
1806/260/244 (H/H) Restless Shadeling