Flayer #5! You can use the sliders to look through the various possible stat configurations on this site here
, or on Warla's site here.
The flayer has more possible breeds than most, which makes finding a rare of your favorite breed very annoying (I have released literally 10 rare flayers over the past week, it's very sad).
For most pets I favor the highest power version, especially pets with heals or life leech-style attacks. Flayer is actually an exception, because he has multiple abilities that rely on going first to be super effective. Flayer #5 is the fastest, with 325 speed (but only 260 power and 1400 health).
Importantly, not every pet can come in every possible breed. So even among pets with identical movesets (e.g. Disgusting Oozeling, Jade Oozeling, Oily Slimeling), only some of them might have the stats that you want. In this case, Jade Oozeling has the best option imo: breedID 4, which is 1481 health, 325 power and 244 speed.
Some others that I have favorites for:
Nether Ray Fry, breedID 4, 1319 health 341 power 260 speed
Ghostly Skull, breedID 7, 1627 health 305 power 228 speed (there's no breedID 4 for this pet, or else I'd want that)
Baby Ape, breedID 4, 1400 health 325 power 260 speed
Molten Hatchling, breedID 18, 1238 health 305 power 305 speed
Restless Shadeling, breedID 6, 1806 health 260 power 244 speed (no good choices here actually, so it doesn't really matter)
Jade Oozeling, breedID 4
Lost of Lordaeron, breedID 3, 1563 health 293 power 244 speed (also no good choices)
Spawn of Onyxia, breedID 4, 1400 health 341 power 244 speed (this one is just miles better than the other breeds)
Minfernal, I would prefer breedID 4 if you're using Meteor Strike, and breedID 6 or 7 if you're using Explode.
Darkshore Cub, breedID 4
Flayer Youngling, again this depends. If you want to beat up critters, then breedID 4 or 6 (high power or high health). For pvp, breedID 5 or 8 (fast and weak, or moderately fast and strong).
Sporeling Sprout, I like breedID 4 or 6. Like Flayer, having a lot of health plays nicely with the humanoid racial, and Sprout has DoTs that trigger it more often, so your return on health is very high.
Fel Flame: there's no breedID 4, so in general I'd go with breedID 7 to get the highest power. The tanky breedID 6 is very good for killing mechanicals though.
Nether Faerie Dragon, breedID 4, this guy is brutal since his Flying racial ability lets him go first despite being only 260 speed, and he has 341 power.
Crimson Geode, I like breedID 4 (358 power!) but it's almost overkill with his burst abilities: the breedID 6 still has 293 power and quite a bit more health.
Blighthawk, breedID 7 has the highest power (305) and decent health -- they don't get any faster than 273 speed, so there's not much point aiming for a 'fast' one.
For more common pets, there are also some special snowflakes. For example if you want a crab with the highest power, you need an Emperor Crab (358!). For cats, Snow Cub (325--Sand Kitten is next best); for the shale hatchling, Emerald and Amethyst have 325 power versions. For Spiders, you'd want a Widow Spiderling or Twilight Spider, they have 341 power versions. For snakes, Tree Python has the top power.