Lets go the opposite of some of the other threads here, such as “worst TV show decay” and Big Bang Theory.
Are there any shows, especially sitcoms, that quit while they were still on a high note, instead of going several seasons too long and getting dropped because of low ratings?
The Sopranos?
Loads of British comedies
Fawlty Towers, Blackadder, Phoenix Nights, The Office etc
Seinfeld comes to mind first. You could argue that the last couple of seasons were formulaic, but they were still hailed and he was looking at a huge contract for more seasons. The finale may not have been all that, but in general I think of Seinfeld as staying good to the end.
Justified
If you are going by ratings and popularity, Seinfeld and Lost would apply. Creatively? Not so much.
On broadcast TV I can’t think of a single one. Some shows were cut off while still doing moderately well in the ratings, but it’s just downright un-American not to milk a success for all its worth.
ETA:
Seinfeld is about as close as you can get. I would have to call that ‘going out on a high note’.
Lost
Heroes
Two and Half Men
Police Squad. I could have watched more; maybe they should have made a movie version of it, with bigger stars perhaps, like OJ!
Mad Men was still a critical darling and topic of conversation up until its end. AMC seemed happy to drag out the remaining season to trawl for awards as opposed to wrapping it up and getting it off the air.
Breaking Bad
I don’t think you read the OP quite closely enough.
Or should I be ducking right about now?
Nope I agree with you.
Men 2.5 may have quit with solid ratings, but creatively it had been in a rut for years.
The Mary Tyler Moore show has always been hailed as a show that went out before it deteriorated, but when I rewatched it several years ago, I disagreed - I thought it suffered the same decay as every other show. Well, except any show that brought in a “Cousin Oliver”, that is. 
They really milked it too far.
Flight of the chonchords. Two good seasons and then the boys called it a day.
The Dick Van Dyke Show
The best of the MTM stable, Mary Tyler Moore and The (first) Bob Newhart Show
Cheers
Barney Miller
E.R. was sagging badly, but it quit before it got canceled, so I’ll argue they knew when to let go.
In season 6, episode 24 of The Mary Tyler Moore Show, they pretty much did just that: Ted and Georgette adopted a son, played by"Cousin Oliver" himself, Robbie Rist. 
JJ Abrams’ shows seem to be pretty good about this. Alias, Lost, and Fringe all lasted about 5 or 6 seasons, and managed to avoid feeling like they were dragging on for too long.
As I recall, they didn’t think they had it in them for a third season. They had used the best of ten year’s worth of collected material for the first season, then got told to create a season two and felt like they were scraping to come up with new quality material that quickly.
I loved Fringe, but I don’t know if it applies here, because the acceleration in plot in the last season was based on the fact that they felt the network was about to pull the plug (and were in danger of cancellation almost every year). I think that’s a separate thing than a show that could have continued on, but chose to end it.
WKRP is a yes and no on this. Yes, because the original run was still running at peak quality when it was let go. No because the revived series stank to high heaven.