I care not for your flimsy rules! Brother Donnie shall be spoken of!
“Chicken Pot Pie…”
Well, if you want to pick someone from The Sopranos, how about the ex-Russian commando who tangled with Paulie and Christopher in the “Pine Barrens” episode?
Yet another nomination from Homicide: Life on the Street - Marcia Gay Harden as the mother of a young boy who was shot in a mall. Holy shit, she had me nearly in tears, and I never cry while watching TV. A very powerful performance.
Dr. Boris Balinkoff, played by Vito Scotti; doesn’t count, though, he was on twice. Once he had rings that could control the castaways minds so he could use them to rob Fort Knox. The other time, he invited the castaways to his island where he used a machine to swap their minds with one another.
He was on two other times as a Japanese sailer who didn’t know the war was over.
And I think “phooey on you” is from the episode with the Russian spy who was an exact double of Gilligan.
Alfre Woodard got an Emmy for her incredible portrayal of a rape victim whose attackers are on trial in the first episode of L.A. Law. It was one of the most memorable performances I’ve ever seen.
Valery? He was actually in another ep–To Save Us All From Satan’s Power. He’s in it very briefly when Tony visits a Russian guy who works with him.
Babylon 5 had quite a few, many of them one-off EarthForce officers who turn up on either side of the Earth Alliance Civil War in season 4.
Of course, you can’t mention awesome one-off characters on B5 without mentioning the Technomage Elric, nor the Centauri Emporer Turhan.
Awesome one-off EarthForce officers:
Captains MacDougal and Hall from “No Surrender, No Retreat”, the two EarthForce captains who we see in that episode (interestingly, Captain Hall is played by the same guy who played Dr. Kelso on Scrubs)
From “Severed Dreams”, Major Ryan was great, finding himself in command after General Hague’s actor ended up signing on for a better-paying two-parter role on Deep Space Nine (the cost of jumping franchises? General Hague got killed between appearances :D)
And of course, General Lefcourt, from the episode “Endgame”. You had to appreciate the principles which led him to be in the position he was in:
Basically, he was commanding President Clark’s forces not because he liked President Clark, but because he felt that Captain Sheridan’s coup was no more legitimate than how Clark came to power. As soon as the civilian government forced Clark out and took power, Lefcourt came to Sheridan’s aid in stopping Clark’s post-suicide Scorched Earth attempt. Also, the dude was just really really good at playing a military commander, IMO. He also appeared in the B5 prequel movie In The Beginning later on, but that was only a couple scenes longer than a cameo.
Dathon from the “Darmok” episode on ST TNG
Rance (Ron’s Dad) Howard had a nice part in which he portrayed Kosh communicating to Sheridan as he (Kosh) was being killed. Rance also played Sheridan’s dad, Kosh had picked that image from Sheridan’s head to communicate with him. Kind of hard to say if that qualifies - Kosh was also played by a couple other people, and was on the show for 3 or so seasons.
Bruce McGill played Major Ryan in “Severed Dreams”, and like you, I thought he was a great character.
Then there’s Zath-RAS. Not to be confused with ZATH-ras. 
Matt Decker from ST:TOS “The Doomsday Machine”, largely because William Windom out-Herods Herod and chews epic quantities of scenery. 
Lloyd Bridges as Commander Kane in the original "Battlestar Galactica in the two part episode “The Living Legend”. Chews the scenery thoroughly and greatly as the ambitious, disobedient, aggressive, ruthless commander of the Pegasus. Plus it features this great dialogue between Baltar and the bubble-headed booby Lucifer, voiced by none other than Jonathan Harris.
Lucifer “One battlestar is approaching”
Baltar (dismissively): “It’s a trick. Who would attack three base ships by himself”
Lucifer “Might I suggest the legendary Commander Kane?”
Baltar (rising from his chair which always had the light shining up in his face, his voice cracking with fear) “Kane?? Yes he would. He is egotistical enough to sacrifice everyone to kill me. Have our base ship pull back and tell the other two base ships to move forward.”
Lucifer “The commanders wouldn’t like that”
Baltar “It is not a request”.
The episode also introduced Anne Lockhart’s cute daughter June to the cast, which had no shortage of cute women. But not much plot or acting.
How about Robin Williams as Mork in Happy Days, a performance so memorable it spawned an entire TV series.
Slight but important correction - June Lockhart’s cute daughter Anne. . .
Thanks to **Marley23 **for taking care of my B5 spoiler in my post above.
Actually, his second appearance was as the same character. On his blind date with Rachel he talked about losing his restaurant because of his drug problem. Monica had set them up. I guess they stayed in touch after the “tartlet” incident?
Laurie was great in that quick role. I also enjoyed Tom Conti as Emily’s father in the London episodes. (*Sort of *a two parter – a cliff-hanger season finale followed by the next season’s premiere.) Best line: “I could kill you with my thumb, you know.”
Jubal Early, from Firefly.
“Well… here I am…”
For those of us who don’t, the first command-level female character shown is the captain of the USS Saratoga NCC-1867 in the opening scenes of Star Trek IV: The Voyage Home, released 1986. There was also Saavik’s Kobayashi Maru training scenario in Star Trek II: The Wrath of Khan, released 1982.
Also from TNG, Ronnie Cox as Capt Jellico.
Yeah, she with an all female armed landing party really kicked those space siren’s asses!
That was my second. My first was Hank Scorpio.
I still laugh when somebody says “hammock.”
Much as it pains me to cut Zathras from ANY list, he appeared in two episodes (Babylon Squared and War Without End) (at least).
Alec Baldwin on Friends, for Phoebe’s great line:
You’re like Santa Claus…on Prozac…at Disney World…getting laid.