[Taqwus]

In terms of being a combat monster, Ambicephalos is certainly rather extreme. He keeps his many sleep vine attacks even if you fill up his three hand slots, IIRC, and two heads + three hands + foot + misc + zero-enc + high hp base allows for numerous interesting options, and he even has recuperation.

If you're more into mages, Orokestes (Arco) and Bart the Patriarch (Pythium) could both be rather nasty given that both nations have a decent shot at a communion machine. Angerboda is much more robust, however.