1. Weeds: Stop after Season 3. Outside of an awesome guest appearance by Albert Brooks in several episodes in the beginning of Season 4 the show goes downhill pretty fast. Without giving away anything, a major event happens at the end of Season 3 that completely changes the dynamic of the show for the worst. You can even tell it's going to suck because they get rid of their awesome theme song "Little Boxes"
2. Buffy the Vampire Slayer: Most fans will tell you to skip Season 1 and they are right. I will go so far as to say don't start watching Season 2 until you get to the first episode where Spike appears.
3. Nip/Tuck: Stop after Season 5 before they leave Miami for Los Angeles. Though some fans will say the show goes downhill after Season 3 but I think it would be impossible for any show to match how incredible Season 3 of Nip/Tuck was. Seasons 4 and 5 are still good but they do pale in comparison to Season 3.
4. Parks and Rec: Start at Season 2. The first season feels like an unfunny clone of The Office. Even going so far as making Leslie Knope a Michael Scott clone. The show doesn't really develop its own personality and style until Season 2 where the characters are more well defined and separates itself from The Office and becomes its own thing.
If you have any suggestions of your own then please feel free to post them