Vader's Frankenstein step with NOOOO, followed by Hayden surfing on the Naboo Fatassed Cow things, Padme combing etc... these are all horrible moments, but what irked me the most is that they blew the most important moment of the entire saga, which is Anakin's downfall.
Anakin fights years with his good old Jedi pals, he hates the Sith who killed Qui-Gonn and who took his hand etc. and from one moment to the next he switches sides and kills innocent younglings? So totally unbelieveable.
The only redemption of the prequels is that without them, there wouldn't be those awesome Robot Chicken spoofs...