Weeds may be the weirdest show on this list (or...any list). The basic premise of the show - skewering suburban life through the eyes of widow Nancy Botwin, a regular single mom who just happened to be a major drug dealer - was solid and flexible that it could have lasted a full run of 5 or 6 seasons without much fuss. And it seemed like that would be the case...until the season 3 finale, which saw Nancy basically burning the town of Agrestic to the ground, abandoning a good chunk of the main characters, and hitting the road to parts unknown. And that's where everything fell apart.
The show was great at doing shocking things and pushing its characters and premise to the very edge, but they pushed a little too hard with the destruction and abandonment of the show's setting. After that, the show grew increasingly ridiculous and unbelievable and turned Nancy from an anti-hero doing what she had to for her family to a straight-up sociopath who was actively destroying her children's lives, except all played for comedy, and following no logical arc. They go from a beach community to living with a cartel kingpin to on the run to a million other locales, none of which were as interesting or dynamic as the simple suburb the show started in.
Seasons Worth Watching: 1-3
Oof. THIS show. So good for two seasons, so Not That Good But I Kept Hoping It Would Regain That Goodness in every other season. Was it losing Half-Sack that killed it? Was it all the Ireland stuff? Was it giving Kurt Sutter so much creative freedom that every episode was like 90 minutes long and there were like 5 montages per episode? Why did Juice live that long? I don't know. I'll never know.
Seasons Worth Watching: 1-2 (there are two solid murders at the end of season 3 that are pretty great, though)
Dexter is sorta the poster child for shows going from critically and regularly popular and then suddenly coming to a screeching halt and everyone unanimously agreeing that the show is unwatchably bad. Dexter was always a PRETTY good show with some weak-ish elements (such as: Dexter's Ghost Dad, Angel's stupid voice, the increasing ridiculousness of no one in Miami PD recognizing Dexter was very obviously a serial killer I mean COME ON), but it was able to deliver true greatness in season 4 with the arc of the Trinity Killer and one of the most gut-wrenching finales of any show up to that point.
And then....uh, season 5 arrived like a wet fart, with nothing even approaching the intensity of the Trinity Killer. Season 6 was the season with Colin Hanks and the most "duh" twist of all-time in the nature of Edward James Olmos' character, and it was at that point that the bad elements of Dexter had truly won out, and left nothing really worth salvaging.
The series went on, and had some brief moments where it looked like things might get watchable again, but then the series finale came and went, and sealed Dexter's fate as a worthless show post-season 4 forever.
Seasons Worth Watching: 1-4 (although you can skip most of the Jimmy Smits season, and any scene heavily featuring Angel/LaGuerta)