The group chat got leaked and now you have been sentenced to death in a Roman wild beast hunt. If you kill the animal, another one will be put in to take its place. You are completely unarmed. Any animal that could eventually kill a human is allowed. (You can’t say something like a fruit fly, for instance.)
Big cats go straight for a quick kill I believe so I’d say a tiger.
Edit: actually I might pick a constrictor snake like anaconda instead. It wont instantly kill you but you’ll likely pass out in under a minute.