EVO - The Search for Eden

In terms of raw damage, either Mammal with Fierce Jaws + Mane (25 bites), Mammal with Horse Jaws, Horse Body, and large (25 kicks), or evolved in Final Ocean to mermaid (25 bites again). If you include Red Crystal forms, the dragon from the cloud maze easily wins out with that 30 damage bite!

In terms of speed, Bird with Longtail birds Tail, Ptenodon's Crest, Pronesaurus Dorsal Fin, and small. Nothing else can reach that 20 Agility! Things that are notably fast with more than 15 Agility are 2-legged small reptiles with mentioned Crest and D-Fin, and Invincible Tail (18), and the mermaid (16). Red Crystal forms are slow, with the eel being the fastest of 'em (12)!

In terms of defense, large Human tops out with 80 HP and 15 defense, though can only evolve-heal spending 1500 for size changes. Notably, a Mammal with Rhino- Body, Bull Horns, and is large has 70 HP and 15 defense, and can evolve-heal the neck for only 200. A reptile can match that 70HP/15Def, but would be intolerably slow and evolve-healing by neck takes 1000. If you include Red Crystal forms, the large elephant from Fort Bird-man in Chapter 4 wins by far, with 200 HP and 20 defense.

In general, I say the dragon is the strongest. Not counting Red Crystals, then mermaid. Not counting forms with single-stage availability, then a large Mammal with Rhino- body, Rhino- jaws, and Bull Horns. Large human is close to the mentioned mammal in terms of stats, but it's slower and doesn't actually hit any harder than the mammal's tackles.