For those unacquainted to the term...:
Flanderization (http://tvtropes.org/pmwiki/pmwiki.php/Main/Flanderization) is "the act of taking a single (often minor) action or trait of a character within a work and exaggerating it more and more over time until it completely consumes the character. Most always, the trait/action becomes completely outlandish and it becomes their defining characteristic. Sitcoms and Sitcom characters are particularly susceptible to this, as are peripheral characters in shows with long runs."

For example, on Friends, Joey went from a little dumb, but otherwise normal at the start of the show, to a blithering idiot who can't add or subtract or tell left from right at the end. Monica went from being a classic type A personality to a neurotic control freak. Chandler went from a normal straight guy who enjoyed beer, pizza and Baywatch but was occasionally mistaken for being gay (http://decider.com/2017/08/08/gay-panic-sitcoms/) to a ridiculously feminine man who hates the NFL and made out with guys in college.

So what shows avoided Flanderization most successfully? What sitcom kept their characters well-rounded and "normal" over the course of multiple seasons?

A better thing to do might be to name comedies full of characters who became more blithering as the seasons went on. Newhart ...Joanna and Dick were the only normal people.

As posed by the OP...I'd have to say, the older comedies, Leave it to Beaver and Father Knows Best were populated by normal, stable adults and kids.

Silly me thought that "Flanderization" must have something to do with this guy.

http://www.troll.me/images2/ned-flanders/ned-flanders.jpg

A better thing to do might be to name comedies full of characters who became more blithering as the seasons went on. Newhart ...Joanna and Dick were the only normal people.

As posed by the OP...I'd have to say, the older comedies, Leave it to Beaver and Father Knows Best were populated by normal, stable adults and kids.
Agreed on all counts. I'd say LITB, FKB, Andy Griffith (first 5 years), and Dick Van Dyke, among others, would fall into this category. They seemed to be more "serious comedies", in the sense they were more grounded in reality and were able to invest more time in the depth and consistency of characters.

In the opposite category, Newhart, as mentioned, is a good example. At the extreme though would have to be Green Acres. By the later seasons practically all the characters had lost all sense of logic, sanity, and reality, but in the earliest episodes this was not the case.

Silly me thought that "Flanderization" must have something to do with this guy.

http://www.troll.me/images2/ned-flanders/ned-flanders.jpg

Ned Flanders was originally a nice guy with a family who was perfect in every way that the Simpsons were flawed. Several years into the show's run, he turned into an quite different character, a holier-than-thou caricature of the Christian Right.

Eric on Boy Meets World. He started out smart then as the seasons went on, he just became dumb as a brick! It was embarrassing to watch him.