I don't know if this has been mentionned, but for me one of the worst sequels are Dragon Age 2 and Prince of Persia.
Dragon Age Origins is one of my favorite game of this generation and brought me back to the RPG world, while the Prince of Persia trilogy brought me to gaming in general.
Dragon Age 2 was, IMHO, garbage and Prince of Persia was the most repetitive game I've played so far with a shitty ending.