In my experience, anyway;
Mimics - The peculiar ability to give themselves nearly impregnable defenses while sacrificing none of their offense makes these things practically invincible. Their only WS being an instant Stun with a strong poison is a help too. Only real weakness is endispel.
Chigoe - Attacking faster than anything else, Chigoes can win a fight before their enemy even hits them once. Weaknesses include low HP, Phalanx, and lack of WS.
Rocs - High HP and attack, and one of the families that can play NIN well. Immune to sleep. Weak WS. NIN job tends to trigger Sneak Attacks constantly.
Wyverns - A WS for almost every occasion, coupled with fast FP growth. Weaknesses include low starting FP and the difficulty of accurately triggering the WS you want.
Weapons - Strong mages with useful feral skills. Weaknesses include general mage weaknesses like low defense and interrupts.
Apkallu - Amazing possible innates and fairly strong WS. Weaknesses include low attack and phalanx.
Rabbits - Whirl Claws is probably the single best damaging WS in Pankration, having high damage normally and occasional huge spikes. Rabbits also have fairly good stats and FP.
Ghosts - Strong spells from BLM and Ectosmash. Aside from doing a lot of damage, Ectosmash cannot be countered by running away like other WS, and seems to be magic rather than physical damage. Resistant to sleep. Cons are low HP, and general mage weaknesses.
Edit: Added Ghost, edited Roc pros/cons.
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