Ankylosaurs please. Give me an armor plated tank with a built in beat stick that could knock a midsized car several feet with a single swing.

If we are going ancient life in general I will always pick Dunkleosteus. A swimming arrangement of thick bone plates the size of a medium bus with a set of Jaws of Life (Death?) for a mouth. The damn thing could slice through or crush literally anything in the ancient oceans and shallow seas. If it weren’t for the Hangenberg event Sharks and Orca would definitely be having some competition.