Some of us have been around longer to offer suggestions but here are some of mine:

2005 East Final loss against the Montreal Alouettes -- there were 44,000 people in the stadium and the Argos got out to an early lead, and then the Alouettes woke up (perhaps inspired by an ill-timed R. Jay Soward TD celebration)

2007 East Final loss against the Winnipeg Blue Bombers -- the Argos were hosting the Grey Cup the next week and laid an egg! This loss was (at least on the field) the turning point for the C&S ownership regime

2013 East Final loss against the Hamilton Tiger-Cats -- the Scott Milanovich regime was never quite the same was it?

Pinball's last game in 2000 -- sad that he had to go out in a blowout loss against the Roughriders