Not really a recurring character, in that he was only in two episodes I know of, but I nominate John Cleese on Third Rock. I wonder why no one has cast Cleese and Lithgow in a production together since then (or before, for that matter.) They have amazing natural chemistry, and the alien characters were a perfect showcase for both of them. The main episode, ending with Cleese melting Lithgow’s human brain, was absolutely hysterical. I can never walk on to a golf course without thinking of bringing an air horn with me.
Marsha Mason as Martin’s girlfriend on Frasier.
The champion, of course, is Martin Landau as Rollin Hand in Mission: Impossible. He made the show.
(Yes, I know he was in every episode for several years. But due to contractual issues, he was always billed as “Special Guest Star.”)
How about Bernard Fox as Dr. Bombay in Bewitched?
Ernest T. Bass on The Andy Griffith Show
I also enjoyed the two or so episodes with Charlene Darling and her family.
Frank Middlemass as Rocky (Lionel’s father) on As Time Goes By.
Despite the fact that she appears as much, or almost as much, as most of the other regulars, Marsha Wallace always gets mentioned as a guest star for her role as Mrs. Krapbabbel on the Simpsons.
Brian Stokes Mitchell as Cam Winston, the upstairs neighbor, on Frasier. One of my favorite sitcom scenes of all time: he and Frasier were feuding over their parking spaces, which led to Cam draping an enormous American flag from his balcony and obscuring the view from Frasier’s. Cam gives an impassioned speech on national pride and ends up leading the apartment board in a round of “America the Beautiful”; Frasier tries the same thing and an old geezer hollers from the back, “Sit down, Crane!”
Not to mention the episode where their parents start dating. Golden, all of them!
I always loved the Q episodes of TNG.
And I’ll second Adam from Northern Exposure. I’m surprised it was only 11 episodes, actually. Just goes to show how well he played the character, that so little screen time made such an impact.
As long as we’re not restricting ourselves to just sitcoms, there’s only been one recurring character who made me giddy with excitement whenever her name would show up in the opening credits, and that was Emily Procter as “Ainsley Hayes”, the blonde republican sex kitten, on The West Wing.
Oh Ainsley, I miss her so.
Who played Frasier’s agent? She was great.
The show itself was borderline—or perhaps over-the-line—wretched, but I rather enjoyed Robert Donner as Exidor on Mork & Mindy. May have something to do with the fact that I worked rather closely with someone who exhibited many of the same behaviors.
Miguel Ferrer as the cranky-ass FBI agent (Albert?) on “Twin Peaks”.
There are two that I haven’t seen mentioned yet, that are personal favorites of mine :
Colleen Dewhurst on Murphy Brown as Murphy’s mother Avery. When you saw Avery, you saw why Murphy was Murphy. Avery was a force of nature, a combination of Eleanor Roosevelt and Mame Dennis, with a little Gypsy Rose Lee thrown in. Colleen did a great job of bringing her to life.
Shelley Winters as Nana Mary on Roseanne. I love the show. Great cast, excellent writers (except for most of the last season), and a nice roster of great guest stars, including Shelley, wonderfully cast as Roseanne’s smart free spirited Nana Mary.
Harriet Sansom Harris.
I’ve liked her in everything I’ve ever seen her in.
My favorite episode was when she and Frasier hosted a local morning show and she became a {local} celebrity for the first time. She was also great in a two episode role as the battered wife of the deceased in Six Feet Under.
Not a sitcom, though.
I came in to mention John Astin as Buddy in Night Court., but Kat beat me to it.
So instead I’ll toss out Allan Arbusas Sidney Freeman on MAS*H.
(huh? Wha?)
Ok, dammit, Mr. Blue Sky beat me to that one! You people all suck! And type too fast. 
Going way, way back for this one, but Hans Conried as Uncle Tonoose on “The Danny Thomas Show,” in the late 1950s and early 1960s always brought an extra level of excitement to the episode.
And where are all the other shows about Lebanese performers these days?
Or Cuban performers, a la Desi Arnez, for that matter?
Along the same lines, Ann Morgan Guilbert (Millie, the wacky neighbor on The Dick Van Dyke Show) never signed a contract because she figured out she could get better billing and a better paycheck as a guest star than she could as the 6th-billed featured player.
Whoever played Daphne’s brother on Frazier was very funny.
Is Harriet Sansom Harris on Desperate Housewives now? I could swear she’s one of the main supporting characters.