For me, it's the 2016 CFP semifinal vs. Clemson. 

The 2013 B1G CG and 2015 MSU in the Shoe tie for second place.

2017 Iowa and 2018 Purdue are serious WTF'ers and deserve some intense, investigative journalism.

As reference: https://www.elevenwarriors.com/forum/ohio-state-football/2019/12/109914/meyers-greatest-game