If we're going for fictitious animals, baggsie Alien. Totally unbeatable - except if trapped in a small escape pod containing it, a tired, emotionally-drained and semi-naked person, and a small cat. Oh.
Honestly, its a huge can of worms. I think fictitious animals / creatures would really need an entirely separate competition of their own.
Even the mighty mantis shrimp is going to struggle against GODZILLA or somesuch beast.