Lucy Ricardo was my instantaneous reaction.
If “greatest” = most laughs per airtime, it would have to be Jim Ignatowski (Christopher Lloyd) on Taxi.
TV Guide agrees with you, according to Wikipedia.
Jerry Seinfeld. No leading character has ever dared to portray selfishness as an attractive quality before!
Me too. I vote for Kramer.
Other Brother Darryl. Neither Darryl ever spoke a line, but the characters defined the whole town. One repeated joke throughout the series, and it never got worn out.
Okay, I couldn’t help it. It was the first thing I thought of.
The lead character and the supporting character have different responsibilities on a show. The supporting character is usually allowed to be more outrageous, and therefore, more memorable.
For lead characters, there are only two contenders – Archie Bunker and Hawkeye Pierce. Lucy Ricardo made the show, but her character was cardboard compared to the other two.
For supporting characters the list is a lot longer. In no particular order I’d throw in Barney Fife, Fonzie, Louie DePalma, Sue Ann (from Mary Tyler Moore), Jim Ignatowski, Niles Crane and Kramer.
I actually think George Constanza is the standout character from Seinfeld. He’s the real everyman. A petty, self-absorbed, jerk loser.
I think Barney Fife has to go pretty high on the list too.
If you count animation, I think it’s hard to top Homer Simpson.
Kramer
Dwight Schrute
David Brent
Basil Fawlty
Edmund Black Adder(am I first to say this?)
Gob Bluth
My favorites have been mentioned:
Barney Fife
Jim Ignatowski
Cosmo Kramer
Louie DePalma
Most of the nominees are supporting characters. I don’t think that is a coincidence. Although Archie Bunker would in the top of my list.
For Ladies: I always enjoyed Jackie Harris (Roseanne), Rhoda Morgenstern (on the MTM show, but not on her own show), and Edith Bunker.
Larry, Darryl and Darryl deserve some consideration. Larry has a line that is one of the most memorable lines in sitcom history.
For a child actor, someone mentioned Eddie Haskell. I think that is a good choice.
ETA: looks like I agree with some posters who posted why I was typing and on the phone.
There is no question about this.
- Lucy Ricardo (from a show I loathe)
- Ralph Kramden
- Archie Bunker
- Homer Simpson
All the rest are also-rans at best.
Ditto.
Fred Sanford. One thing I loved about the character is that while he obviously loved his son and his late wife, he didn’t have the obligatory “Heart of Gold” underneath (nuggets of gold in his heart perhaps); he’d con his own son in a second in order to get something he really wanted, but then would get furious if anybody else tried to put the “dummy” down. He was also a lot sharper than most (including his son) usually gave him credit for, being able to smell B.S. 20 miles away, as expert in never missing a meal without ever having to sweat as Kramer e’er dared be, and as shrewd a social commentator as Archie. And while he was bad and selfish and sly enough to be interesting and funny, he never did anything really bad.
Of course it’s hard to divorce the character from the impeccable comic delivery and timing of the actor (whose previous acting experience was negligible as were most of the other great cast members on the show). But then, that’s true of any great character; if Archie Bunker had been played by Mickey Rooney as once suggested it’s doubtful he’d be that remembered.
After some consideration, I think Dick Solomon and Sophia Petrillo should also be thrown to the mix.
Archie Bunker. Although now that I’m older and have some real-world experience I’ve come to realize he wasn’t as stupid as I thought.
So many people naming Archie Bunker, and he’s pretty much unknown in the UK. 
Weird how the occasional big cultural icon doesn’t get transferred to such status in both countries, even though most do.
Frasier was the first that came to mind for me. I preferred the other characters in his show, but he was the one that it got started for after Cheers.
I came to vote for Louie De Palma butI admit Archie Bunker is a string contender.
I toss my vote in for Bill McNeal from Newsradio.
Consistently great
Well, someone upthread did mention Alf Garnett, Archie’s English predecessor, though I don’t know if he was either as good or as memorable a character as Archie, never having seen Till Death Us Do Part.
Some honorable mentions:
Jack Tripper
George Jefferson
Maxwell Smart
Oh, he was excellent - and like with Archie Bunker (I just wikied him), the writers and the actor were saddened that so many people were laughing with him, not at him, as was their intention. He had a great many moments of pathos in amongst the ranting. I’d also say that most people would know the name even though it’s been off TV for years.
Trivia: his son-in-law was played by Cherie Blair’s dad.
Sgt Bilko