The worst actual ride system is Jungle Cruise. The cruise guides make it entertaining, and I actually enjoy it for the combination of bad jokes and kitchiness, but the ride itself is an embarassment. It's hard to believe it's only 33 years old!
The worst show has to be Circle of Life in the Land Pavilion. It is a morality tale about how overdevelopment can ruin the environment, which is not so bad in itself. But it has little entertainment value aside from using Lion King characters, and the irony of WDW itself being the cause of the most of the (over)development in central Florida makes it hard to swallow. When we saw it there were 6 (count 'em, six) people in the auditorium, and 4 of them were us! Makes the Country Bears look like Garth Brooks.
That monorail at the airport is kinda lame, too.