In my own time, the worst moment as a Leaf fan was January 1997.
The Leafs had traded Eric Fichaud to the Islanders in 1995, and Kenny Jonsson and what turned out to be the Roberto Luongo pick in 1996 to get Wendel Clark back. In January, the Leafs were dead last in the standings and the Islanders were second last.
On the radio one night, they talked about how terrible it would be for the Islanders to come into Toronto in the future with a Thornton/Marleau duo up front, Kenny Jonsson manning the blueline on defense and Fichaud in net. They talked about how the Leafs had basically signed away their future and allowed the Islanders to have all our glory for years to come.
That was kind of a low point from a fan's morale point of view.