Getting to the SB and losing it. After climbing the mountain to get there, missing the opportunity is the worst, IMO.
Next, is losing the Conference Championship and a chance to get there.
Being out of it in December or losing a Playoff slot on a tiebreaker are both bummers, but at least you have no one but yourself to blame for a whole season's play and can't wonder "what might have been" if you had been able to make ONE MORE G*dd&^%#D FIRST DOWN IN THE FOURTH QUARTER AFTER YOU WERE UP BY THREE TOUCHDOWNS IN THE AFC CHAMPIONSHIP GAME...but, I'm over that now...it doesn't bother me any more...nope...I'm fine...it's all in the past...I've moved on...forgotten all about it...%$#@#$%^