I always liked Richard in FRIENDS (played by Tom Selleck).
Probably just because I like Tom Selleck.
Garak and Dukat on Deep Space 9. The show always picked up when they were around and being morally ambiguous or downright amoral. Especially when they showed up in the same episode and had one of their hissing Cardie cat-fights.
Just a one shot deal, but the guy who thought he was a werewolf on Barney Miller was pretty great. I still like to say, “I’ve got to to to worrrrrrrrrrrrrrrrrrrrrrk!”
What’s that actor’s name?
Will and Grace has had some of the best guest appearances; Michael Douglas as the gay detective, Matt Damon as the straight guy trying to get into the Gay Man’s Choir, Minnie Driver as Stan’s mistress…I know many of you despise this show, but these guest performances were a scream.
William Shatner on 3[sup]rd[/sup] Rock From the Sun. Inspired. And the Twilight Zone in-joke had me in hysterics for the rest of the episode.
He appeared on Barney Miller more than once.
Kenneth Tigar … Mr. Kopeckne
“Barney Miller” playing “Mr. Kopeckne” in episode: “Werewolf” (episode # 3.6) 28 October 1976
“Barney Miller” playing “Mr. Kopeckne” in episode: “The Ghost” (episode # 4.13) 12 January 1978
Then to really confuse things:
“Barney Miller” playing “Jesus Christ” in episode: “Vacation” (episode # 6.3) 27 September 1979
“Barney Miller” playing “Philip Pollock” (as Ken Tigar) in episode: “The Psychic” (episode # 7.11) 5 February 1981
One more time as Mr. Kopeckne
“Barney Miller” playing “Mr. Kopeckne” in episode: “Possession” (episode # 8.4) 19 November 1981
One of TV’s great character actors.
Jim
I love the CSM episodes! I also liked the Lone Gunmen and Alex Krycek (it was fun watching him get beat up every time he showed up in an episode).
One more for Barney Miller:
Florence Stanley as Bernice Fish was great.
They were way ahead of their time, too. They were played comedically (because it was a sit-com) but at the same time seriously. One episode involved the more normal of the two (i.e. the one who wasn’t a kleptomaniac) arrested for kidnapping his son from an ex-wife who wouldn’t allow him to visit his father due to his new gay “lifestyle”; during the episode Wojo actually came to side with the gay father. That was 15-20 years before most primetime shows would have a plotline that sympathetic to a gay father.
Sanford & Son had some of the greatest recurring characters of any show: Bubba, Achu, Julio, etc., with one shot deals by Slappy White, Scatman Crothers and other Chitlin’ Belt legends. Even Grady and Aunt Esther weren’t in as many episodes asa you might think (except for when Grady filled in for Fred during his St. Louis trip/Redd Foxx’s contract dispute and proved so popular Foxx came back), but I’ll count them as regulars for this.
It’s to early to know if this character will return, but I have high hopes that he’ll be around some more. Billy Gibbons (ZZ Top guitarist, the one with the beard) played himself as the father of Angela Montenegro, the artist on Bones.
Angela’s dad is Billy Freakin’ Gibbons!!??!
I always liked seeing Joe Walsh on the Drew Carey Show too.
Shelly Winters as Nana Mary on “Roseanne.”
Damn, the Lone Gunmen were already taken.
Ummmm. How about Lady Heather on CSI?
Another vote for Mandy, 24’s terrorist lesbian assassin. Three appearances in four seasons, and she still elicits a “Holy shit, it’s Mandy!” from everyone.
Did I miss TNG’s Q?
What you didn’t like Troy McClure on the Simpsons? 
BTW someone did say Q earlier on.
http://boards.straightdope.com/sdmb/showpost.php?p=6904226&postcount=3
I agree with many already mentioned, and I’ll add: Eddie LeBec from Cheers.
Wow, nearly every character I came in to mention has already been listed.
Leonardo Leonardo on Clerks: The Cartoon Series, voiced by Alec Baldwin.
Patrick Warbutron’s appearances on Newsradio were very funny.
Oh man, I nearly forgot!
Zathras!
“Is a sad life for Zathras. But probably have sad death, so at least there is symmetry.”
I loved Ernest T. Bass on The Andy Griffith Show, the Darling family also.
Jerry Van Dyke on The Dick Van Dyke show.
The guy who played Chandler’s roommate on a few episodes was great too.
While we’re on the subject of Andy Griffith, how about those two “fun girls” from Mount Pilot.
Damn, most of mine were mentioned…
So, I’ll second (third, fourth, whatever)
Harry Anderson as Harry ‘The Hat’ Gittes in Cheers
John Astin as Buddy Ryan in Night Court
Brent Spiner as Bob Wheeler also in Night Court
David Cross as Donnie in Just Shoot Me
and I’ll add:
Olivia D’Abo as Nicole Wallace in Law & Order:CI