Norm Petersen and Cliff Clavin.
And Professor Frink. Moyven.
If that’s your criteria, it would have to be Jack Benny.
Come on . . .
Only one mention of Ralph Kramden (Jackie Gleason)??? For decades there were only 38 available known recorded shows yet in any major city you could tune into one of those episodes on any night of the week. The only sitcom to come close to that longevity is Seinfeld and they recorded that number of episode in many multiples.
“The Honeymooners” define sitcom.
Fonsy (sp) from Happy Days is right up there with Archie but also Louie, Jim, Hawkeye, etc. Too many good ones with too many memorable moments that get “Mention of Honor”.
Nobody has mentioned Frank and Marie Barone, two characters whom people love to hate, and hate to love.
But the all-time champ has to be Archie Bunker.
Can’t believe no one has mentioned the legendary Captain Tuttle.
Dan Conner from Roseanne makes the list for me, I empathized with that guy a great deal. John Goodman is a far better actor than the roles he’s been offered (with exception to Walter Sobchak).
I’d also throw Cliff Huxtable out there.
There is a little Captain Tuttle in all of us. In fact you could say we made up Tuttle.
I was discussing this with my sister, and we think that the entire Bluth family needs a mention, but if I was forced to choose one, I’d go with GOB.
Sir Humphrey Appleby from Yes Minister/ Prime Minister
I came in here to mention Fred Sanford, but since you stole him from me, I will nominate Dorothy Zbornack. I gotta wonder if she would crack me up as much without the other golden girls with her. They crack me up too.
And I agree that Archie Bunker was great.
Best support…Baldrick
If youre talking about sitcoms being long-running, then Last of the Summer Wine beats any other contender ad Nora Batty would be the greatest ever character. The show hasn’t officially been cancelled and there might be even more seasons of it, despite the way the actors keep dying off. 
From MAS*H
Colonel Flagg. I loved that character.
Archie Bunker for all the reasons listed.
Al Bundy for playing the completely beat down, middle aged man who still clings to his one moment of glory. A touchdown he scored for Polk High.
Animation:
Homer Simpson
Ned Flanders
Quagmire (giggidy, giggidy)
Nelson (HA HA!)
Another vote for Archie Bunker. First name that popped into my head upon seeing the thread title too,
You and Bup got mine. Deputy Fife forever!
Just thinking about Basil Fawlty makes me smile though.
I think comic actors deserve credit for allowing their characters to seem utterly ridiculous and even unlikable. Those are two great examples.
I am glad that this thread moved so quickly from the “My favourite character in a current show I like” to an actual contemplation of Great Characters.
Like in about one post.
I can’t see Captain Tuttle as one of the great sitcom characters of all time. In fact, it’s already hard to remember what he looked like.
No one has mentioned Felix Unger yet, so I will.
Another vote for Eric Cartman.