Technical answer; a balanced cast with creatures capable of human thought. I mean you're not going to pit werewolves against ghosts in combat and no matter how cool a Roc might be, they're not going to be great to interact with. So for this you will probably want to set a standard with one creature and don't let your other creatures deviate too far from that curve, or adjust these creatures with magical GM powers so it's more balanced.
Things I have really enjoyed playing-answer; I loved the Tarrasque, a hivemind of insects capable of combining into human form, a fire-breathing, six-limbed lovechild of a dragon turtle and a bear, a centaur and a djinn. Not necessarily in that order. Granted though, many of these were heavily altered or by my own creation