Aw, don't tell me Stargate starts to suck in its second half. I just started watching it a few weeks ago and was looking forward to ten great/good seasons.
You get 6 great ones, 2 good ones, and two (the last two) that shuttle between mediocre and dire. But these last two also are essentially a completely new storyline, so you get to finish the original SG-1 arc within the good seasons.
SG-1 definitely should have ended earlier. Moebius should have been its series finale. I'd even have been quite keen on made-for-DVD movies afterwards.
Buffy should have ended with the 5th season, really. There is plenty I did enjoy about the last two years but looking at the series as a whole, it would be stronger without them.
The Simpsons - Season 9, maybe 10 with only limited drop in quality. After that, it's junk. And they did the movie about 8 years too late.
Friends ended a year too late.
ER should have ended with season 10, imho.