I could've sworn we had a thread like this already? Or perhaps it was just a question posed in the "Would You Rather" thread.
At any rate...
1. Attack of the Killer App
(the worst episode of the series, relied almost solely on present-day references and gross-out humour, the Susan Boil thing was atrocious, the sub-plot with Mom went nowhere and the whole thing was just fucking painful to watch)
2. All the Presidents' Heads
(shitty excuse for a time travel episode, something Futurama had done perfectly up to this point... barely made sense and lacked too much in the comedy department to make up for that)
3. The Futurama Holiday Spectacular
(completely unnecessary, and unfunny to boot)
4. Saturday Morning Fun Pit
5. A Leela of Her Own
(ruined Blernsball, one of the funniest jokes the series had going for it; also boring and unfunny)