That's a hard question.
I loved Scrubs, but it reached it's natural conclusion and I saw the horror that was their attempts to give it a 9th season.
Lost was great, but it 'lost' itself somewhere during season 3 and ended up having one of the most sentimental horseshit endings ever.
Buffy rocked, but it also reached it's natural conclusion and it didn't really have anywhere to go after that - the comic books proved that much.
Most of my favourite programmes actually went on too long. TV is better when it's done for arts sake rather than for financial gain. When TV shows become too popular, they string them out far longer than they actually have content to fill.
I guess I'd probably bring back Don't Trust The Bitch In Apartment 23. That probably ended too soon and I kind of enjoyed it.
Longest game answer ever...
Have you ever typed something so long and boring that you actually deleted it rather than posting?