the worst fate possible would be to be reincarnated into an animal that is perpetually smiling. So even if i were bummed out, having a crappy day, id still be smiling. I can only think of goats, and dolphins tho as animals that fall into this category (of always smiling).
and I dont believe in reincarnation. but it would be weird to be reincarnated into another human being born, minus our former experiences, thus an endless cycle.