The breakup of the Bulls dynasty is right up there for me, as well.
And every year until this year. They were terrible, forgettable, etc.
1990(?) NFC Championship game is also up there. 49ers vs Giants at the 'Stick, the Giants win with a last second FG sending them to Super Bowl XXV to face Buffalo.
1995 NFC Divisional playoffs - Adam Walker's fumble recovered by the Packers which killed the 49ers. They never recovered from it.
Just about any time the Packers beat the 49ers. Those were horrible.