Leaving on a high note?

Last week, we discussed what shows we might have saved from cancellation if we had my allegedly mighty "Chuck"-saving abilities (along with the power to travel through time). 200 comments in, Film Cricket made this observation:

Mostly, though, I'd use my time-traveling TV powers to end shows that dragged on beyond their Best Before date (I guess in the hope that newer, better things would be programmed?): "Friends," "Seinfeld," "The X-Files," "Heroes," "ER," etc.

And that leads to this week's question for you all: what one show would you end prematurely to preserve its genius, and when? More thoughts, including my own pick, coming right up...

There's something to be said for watching your favorite show as long as you can, regardless of how much the quality dips, but there's also something to be said for a show like "Freaks and Geeks," which did 18 episodes and then went away while our memories of it were still perfect. As George Costanza learned, sometimes you're better off saying, "That's it for me!" and walking out while your audience is still happy.

Once upon a time, I would have used a show-canceling power on "Friday Night Lights" after the first season finale, particularly after how the second season began, but then parts of the third season were so great that I'm glad the show continued, even if we had to suffer through The Unfortunate Incident of the Beer Bottle in the Nighttime.

So my pick would be "Homicide: Life on the Street," probably at the end of the third season, or at most midway through the fourth season (after the introduction of Kellerman, plus "Doll's Eyes," but before we met Dr. Julianna Cox). The longer "Homicide" ran, the less resemblance it had to the show I fell in love with. The cases became more hyperbolic (snipers! arson! eeeeeeeeevil drug lords!), the new characters were for the most part more attractive but vastly less interesting than the ones they replaced, Andre Braugher clearly started to get bored (and then left), etc., etc. I'd rather not have my memories of "Homicide" clouded by the likes of Falsone, Rene Sheppard and Luther Mahoney, you know?

What would be your choice, when and why? I don't want this to just be a Jump the Shark thread -- the show in question could have just become less good without putting the Fonz on waterskis, or without letting a Sherman-Palladino or Sorkin walk out the door.