Besides shows that were canceled prematurely/unwillingly, which shows went out on their own terms exactly when they should have?

Big Bang had a few more seasons left IMO.

Jim Parsons was the one that wanted to leave...

Penny/Kaley was/is still HOT !

Matlock had 3 more Seasons when it moved to ABC after NBC decided not to continue airing it in 1992

Newhart ended at the right time on CBS back in 1990 after 8 years with over 180 episodes

Cheers did get a proper finale at the end of the 11th Season on NBC in 1993

The Original Knight Rider completed its 4 year run in 1986

WKRP in Cincinnati went off the air in 1982 as the show was this close to reaching the 100th episode milestone

leave it to neaver and the brady bunch both ended at the right time

The Partridge Family left the airwaves in 1974

Petticoat Junction ended in 1970 on good terms, same thing happened to The Andy Griffith Show in 1968

Charlie’s Angels finishes it run in 1981 the same year Dynasty, Falcon Crest & Hill Street Blues all first aired

The Mary Tyler Moore Show ended a healthy seven year run.

Barney Miller ended after 8 seasons (season 1 was a mid-season replacement with 13 episodes).

Susan Harris stated that "Soap" was supposed to have lasted five seasons but was taken off in four. For me as a fan of the series, it didnt drop off in humor or quality, it was a stange vocal mix of Christians and militant homosexuals which killed it off.

The number one show this may pertain is Best of the West, 1981 on ABC. in my opinion the most laugh out loud, hilarious sitcom of all time, cancelled in one year.

Cheers
Family Ties
Home Improvement
How to Marry a Millionaire
The Brady Bunch
The Mary Tyler Moore Show
The Golden Girls
The King of Queens