The winner is...
This ecosystem includes the Velociraptor, Protoceratops Deinocheirus mirificus, Plesiohadros, and more!

Image by PaleoGuy on Deviant Art.
This ecosystem includes what YouTuber Best in Slot refers to as "danger floof," or the Therizinosaurus, which is my favorite:

Image by Sergey Krasovskiy
Now, several people noticed that it ended in a close tie at the end between Mongolia/Flaming Cliffs and Hell's Creek, so we took a look at the IP addresses of the votes to make sure our own office votes didn't skew the results. What we found instead though, was... drum roll... massive voter fraud!
Okay, not that massive, but our top IP address voted 29 times for Hell's Creek!
Now we do recognize some of the smaller ones can be multiple family members or people in the same location, but 29 people with the exact same IP address voting for Hell's Creek with most of the votes only taking 2-3 seconds to complete? Sorry, someone was cheating.
So we dug into the data, removed excess suspicious votes, and by the end, Flaming Cliffs was the clear winner:
And, for your humor, we made this handy cheating infographic -- really all in good fun because it's not like this was a super serious legal vote. But we did want to get to the bottom of what the actual most popular ecosystem would be, and we did!
