So many games, so little time. I would have to echo so many other fans in putting the 2002 Apple Cup at the top of the list. So much was at stake and a triple OT loss is just so hard to take.
I have 3 honorable mentions, but only 3 because I don't want to think too much about the question:
1. 1975 Apple Cup. 27-14 lead with 2 minutes to go and the ball on the UW 14-yard line. Then the pick six & miracle heave by Warren Moon, and that's the end of Jim Sweeney.
2. 2019 UCLA loss. Like many of these frustrating games, it's agonizing to think about "if one play had gone differently". But Cougs didn't deserve to win that one. Enough said.
3. I'll add a new one that probably doesn't belong but it stuck with me for some reason....in the big picture, a very insignificant game but frustrating nonetheless. 2004 loss to the Ducks 41-38. Cougs had a 10 point lead with 8 minutes to go. My wife committed the ultimate sin at that point and shouted "We're going to beat the Ducks!". While I loved her optimism, her grasp on reality as a Cougar fan comes and goes. WSU still had a 4-point lead with 3 minutes to go but couldn't stop the game winning TD. What made it most frustrating was that the Cougs gave up 27 points in the 4th quarter after giving up only 14 in the first 3 quarters. Ducks fumbled SIX times, but WSU still couldn't prevail. I lived in Oregon for nearly my entire life at that time (since relocated) and my so-called Duck fans were more than happy to rub it in with the usual "you Coug'ed it!."
Glad Cougar
Like you said - so many games, so little time. I guess that's the reason for the "Coug'ed It" meme we all hate so much?
In my younger days, a horrible Coug loss would stick with me seemingly forever. Now, not so much. Mostly I am able to put things behind me pretty quickly, win or lose, and move on to the next one. That works for me MOST of the time. But there are still games and circumstances that eat at my gut.
First and foremost is that 1975 Apple Cup. Especially meaningful to me as I had just graduated from WSU and was attending grad/professional school at the UW at the time. A group of us, all WSU grads in the same class/program, had arranged to get the best UW student seats in the house for that Apple Cup.
So, there we were, sitting in the "Gold Section" of UW student seating, cheering our azzes off for the Cougs. No one really bothered us at all. As you described it - 13 point lead with a bit over two minutes left and the ball on the UW 14 (or so?) yard line. OMG - there's no way to blow this one!! Except we did, as you described.
I remember standing there with my group long after the game had ended. Not really believing what had just happened. soaked through my inadequate rain gear, stunned beyond belief. It gets fuzzy after that. I don't remember getting home (renting a duplex on the back side of QA Hill, overlooking the Ballard Bridge). But I do remember that my wife's family was there for dinner and I was like a Coug kitten that had been thrown into the Montlake Cut in a sack, pulled out just before totally drowned. NOT "good company" to say the least.
THAT ONE has stayed with me all this time. It burns a hole in my gut every time I think of the circumstances. WHY throw a pass into the flat, from the 14, the ONLY place it could have turned into disaster? Of COURSE we couldn't get the kickoff and run the clock out after that! They get the ball back and, on the ridiculous TD pass, all Ken Green had to do was bat the damn ball down. Instead he deflects it up and into Spider Gaines's hands. And the GD PAT was wide, but they gave it to them anyway.
I can't go on....the acid reflux is taking over.
Like I said - ALL the others are perfectly valid. I just have been able to put things behind me pretty quickly for most of them. But that '75 AC will haunt me to my grave.