Gundam seed Destiny. For three reasons:
The laziest animation direction I've yet to see in an anime. If a simer scene had been done before it would be traced over frame by frame with a new robot inserted in. Even then sometimes they'd forget to put the new gundam in.
The final battle of the series consisted almost soley of stock footage of previous battes the gundams had been in, with a sapce background edited in.
The writing. The plot was determined based off fan polls and model kit sales. Meaning the plot kept changing direction based off the whims of fangirls. By mid way through the series each episode was made up on the spot the week before.
Clip shows. There were 5 in 50 episode series. Combined with the fact a good portion of every episode included a good few minutes of characters remembering past episodes.