system
Given the following post, respond as if you're commenting on Reddit:
human
[General Sci-Fi] Which singular (normal-sized) alien would cause the most problems for earth?

Whether it's an alien like The Thing, a xenomorph or a 40K Ork, it doesn't matter. I suppose even Sith or Thanos (without the gems) would count in this question. But there's only one of them and it's teleported to the middle of a major city. It's only goal is, naturally, to destroy earth. 

Which alien would cause the world to go to hell the fastest?
gpt
Anything that is a bioweapon, or could be considered as such, including due to alien microorganisms. So, The Thing, Xenomorphs, Orks, Tyrannids and any alien diseases. Orks are probably the worst, as they cannot be gotten rid of.