Different pets for different tasks but in terms of tanks, nowdays most people will agree:
No. 1 all time best pet in game for most purposes and specs nefore and NOW : --Cusidhe-- make sure he has 590-600 hps untamed, and make sure cold res is as low as possible, pref low 70's. You can make diff specs of Cu, and they are rideable, just careful where you put training points on. Not a squishy pet at all.
No. 2 with new taming : The 2 slot Najasaurus. Has high magic resist, high poison, and can get lethal poison area spells, good for spawns, also you can put FZWW or AI on him.
No. 3 Fire Beetle, for 100% Fire, Rune corruption that drops ALL resists of t he enemy down by 50%, starts at 1 slot, and can be a tank. Blue beetle also for same reason, and massive storage for loot at high strength, and they are both rideable!
No. 4 with new taming: LESSER 1-2 slot Hiryus. Though they are a little squishy, they can get true Bushido, and AI possible Goo, and they have huge innate mana regeneration, the best of ALL pets. Also they are rideable.
No 5 New 2-slot Reptalons, with energy damage and stun, and rideable.
No 6 New FROST DRAGONS with 115-120 wres, 120 parry, other overcaps AND 1,000 HPS, and cold damage.
There are other great pets in game, and more will happen as promised by Devs.
DO NOT pay any money for weird looking stats/pets, or any pets and
do not waste your time on Cold Drakes : They look good on paper overcaps and all, but in real battle they are super squishy, unknown reason. Dont buy one, at least for now.
Dont get Dread spiders : Idea was great, but the gimped product is a failure : NOT enough slots to make this potentially top pet in the game, as it is now it is trashed.
Most importantly:-----VVVVVVV-----
If you have any legacy pets, do not train them yet, do not release them even if their slots look strange.
These are extremely valuable, and irreplaceable. Do not listen to people in game that they are worthless. Post the pets here FIRST and ask opinions here..I hope this helps a little.