I wasn't really a die hard KSU football fan before I came to school here, so while I remember '00 OU, '98 A&M, being disappointing, they weren't really gut wrenching for me personally. Here's my most gut wrenchiest losses.
1. 12 Baylor
2. 03 Marshall
3. 02 Colorado
4. 11 OU
5. 04 Kansas
6. 02 Texas
7. 03 Texas
8. 07 Kansas
9. 10 Nebraska
10. 04 Colorado
I dunno but Bowl games don't wrench my gut quite like a regular season game does. Even the Fiesta Bowls. 05 Colorado, 06 Kansas, 07 Missouri (agree with zacker on this, it was a low moment), 04 Fresno, 05 Nebraska, 09 Nebraska, 13 NDSU, etc were all sad factories also.
I'm kinda in the same boat, granted I wasn't in school until '07, but I didn't follow the 'cats until '00. My list would be:
1. '12 Baylor, for obvious reasons
2. '00 OU (champ game), while the second loss to OU that year, it was the first time I really had my will for the cats thwarted, and it hurt bad
3. '10 Nebraska, by far the worst I ever felt after a game I attended, even if the game ended up being meaningless.
4. '11 OU, undefeated at that point, competitive first half, and then totally crapped the bed in the second half.
5. '07 Kansas, totally ruined my mood for Prince, the season, and why I don't awfully care if they keep having 3-9 seasons for the next 20 years, TSC be damned, also when I realized I don't care how talented JFree was he was an awful decision maker.
6. '03 Marshall, felt like how I felt after the NDSU game, but way worse.
7. '07 Missouri, cheering for them to beat ku looked and felt awful, maybe the most visible low point for Prince, but to me the back to back 40+ losses or w/e to OU and TT in '08 were the low point in terms of the program. That and getting blown out by KU in Lawrence.
8. '02 Texas, at least the Auburn game made that kicking choke job look not as bad, but still.
9. '03 Texas, dat fumble.
10. '13 NDSU, hate seeing some lowly team beat us, at home, and revel in it. And the fireworks at the end just make it all seem like a joke and im sure those NDSU fans thought it was the greatest night of their lives.
And it doesn't count as gut wrenching but '08 Kansas was awful, I turned it off like halfway through the second quarter, earliest I ever stopped caring about the KSU game ever. '11 OSU game was gut wrenching to lose ultimately, but it doesn't make the list, I never thought we'd had a chance and the fact we ended up 5 yards of at least tying and if not winning off a 2 point conversion makes it my favorite game we lost, which is not really much but it was a fun game, lots of back and forth, and high drama.