Lost's Worst Season
Season 6 certainly was (at least for me) the lowest point of the entire series. Worse then even Season 5, which suffered from a lack of direction nearly all season and certainly worse than season 3's aimless early episodes.
In season 6, almost all of the on-island stuff was ultimately unnecessary. Most of the cast sat around for great lengths of time, walked from place to place just to sit some more, and lacked any cohesive direction. Had the story progressed during that time, I wouldn't have a problem, but almost none of the things they did the first half of the season made a single lick of difference.
Character growth on-island was not only stunted, but regressed. All the character growth of the last 3 years (in-universe, off screen mostly) was swept away. It was like they ignored all of season 4 and 5 and were writing these people like they were still in early season 3.