If I had to pick 5 then they would be:
1, Fools and Horses. An obvious choice I know but the episodes in 1996 would have ended the show perfectly.
2, Red Dwarf. Take away half of the writing team and you're only left with half a show. Introducing new characters, making more episodes per series, filming without an audience and remastering the old series were all mistakes in my opinion.
3, Game On. The first series of this show was genuinely superb I think but the same old problems of losing writers and cast members meant a decline ensued. It still wasn't terrible but just not the same.
4, Brittas Empire. Another show that had a natural ending and then ripped it up to make money. The episodes written by new writers are clearly discernable from waht had gone before as well.
5, Blackadder. Back and Forth wasn't up to the high standards set and I hope that they never make a 5th series.
I neglected to pick Summer Wine because I think that it's coped with cast changes better than most sitcoms. I wouldn't have minded if it had ended after Bill Owen's death though.