The superbowl loss to the Patriots stung a lot back then, but it's 3rd on my list.
1st was the playoff loss to the Saints in 2000 - we beat the Saints in week 17 in New Orleans to earn a wildcard spot, and were playing them again the following week in the wildcard round. Our defense was one of the worst of all time that year, while our offense was arguably the best of all time that year. Naturally, I was very confident going into the game. We got down 31-7, but the ripped off 3 TDs in the 3rd/4th quarters to make it 31-28. Our horrific defense got a stop and the Saints were punting with two minutes left. They couldn't stop us and I just know we had won....
....until Az Hakim muffed the punt. Three kneel downs later, game over. This one stung because the teams we would have to face in the NFC playoffs were the Vikings and Giants, and we had beaten them both that year pretty easily. I still believe we would have been in that superbowl against the Ravens, and with our explosive offense against their historically great defense....that would have been something..
2nd was 3 years later...Division round playoff loss to the Panthers. We needed an onside kick and FG to take it to overtime, and we got it - although Martz inexplicably played for the FG in regulation by running the clock down and settling for the FG....anyway, game went to two Overtimes. In the first both teams missed FGs and I thought ours was going in - was already jumping up and down as it was dead on straight......but fell just short.
Then on 3rd and 14, the first play in the second overtime, Steve Smith takes a pass over the middle to the house. I'll never forget the empty feeling watching him run the distance, knowing the game was over and feeling like it would never end.
then after those I would say Superbowl 36. It's still I think the 2nd or 3rd biggest upset in superbowl history, so it should bother me more, but we were losing the entire game and came back to tie it late, which led to Brady's signature career making drive. I had accepted defeat early on in this game, so it just didn't stick with me for long. In hindsight it was the end of the Rams attempt at a dynasty, which sucks, but at the time I thought we would be right back in it. What came out after the game, but nobody really talks about because we're the Rams and St. Louis was too small a market, is that the Patriots held our WRs and Marshall out of the backfield all game long...the refs basically helped rob us of that superbowl as well, and of course there was something about the Patriots filming our walkthrough....but our team partied late the night before and took the Patriots for granted....got what we deserved for that.