1) Losing the Super Bowl in 2001. It's a loss that has stayed with me the longest and most intensely, as it took the perfect storm of bad things to allow it to happen. 1) The Rams taking the Patriots lightly, 2) having the extra week of prep time eliminated due to 9-11 (with the week, the Rams would have come out in a two -minute hurry up and the Patriots would have had no chance), and 3) the refs just letting everything go so that the Patriots could be as physical as they wanted to be. And all that was before the cheating allegations. No other lose is even close.
2) The NFC Championship game in 1989. After that the ridiculous ending to the Monday night game a few weeks before, I just thought "if we can just get these guys again". We did, and started fast before Ronnie Lott saved a TD and an early Rams 14 point lead. The Rams, and Jim Everett just fell apart after that. He went from becoming what I thought he would be against the Saints a few weeks earlier--when he just assumed control and willed them to two TDs in the last four minutes to force overtime and the win, to a complete gutless stiff when he went down with the phantom sack in the Championship game. As it turned out, it was the swan song of the original Los Angeles Rams, as they disintegrated into mediocrity after that before finally moving to St Louis.
3) It's really hard to separate the 1975 and 1978 Championship games, I went in believing we were going to win both against the Cowboys and then things just went wrong--over and over, usually in the form of turnovers that just swung momentum. Whether it was the Cappeletti/Haden 1st quarter TOs in 1978 or the James Harris/Lawrence McCutcheon 1st Qtr TOs in 1975. Both heartbreaking, especially to a teenager.
4) 1976 NFC Championship game against the Vikings. This is actually probably #3, but the two failures against the Cowboys swung the ranking. Driving all the way down the field on the 2nd series, having Ron Jessie score (I will always claim he did) not only be turned away and settle for the field goal try, but to have it blocked and returned for a TD (after going for it on 4th down in the same spot against the Vikings in the regular season) was almost unfathomable. That one play was pretty much the metaphor for the 1970s Rams playoff failures.
5) Trading Dickerson. We finally had a QB to go with the RB and Eric just couldn't be the team player he should have been. If he stays in LA, he may well be the NFLs all time leading rusher, and we may have gotten over the hump. We will never know what could have been.
