I’m having a hard time considering William Macy and Chris Cooper as character actors. I think that both have had enough starring roles to be considered A-list actors.
No love for Steven Tobolowsky yet? He’s seems to have been in every show and movie ever made. 
J.
p.s., Oh, and I forgot to add that his podcasts are hilarious. He usually doesn’t show his comic side in his acting jobs, but he’s a great comedy writer.
Simon Oakland
Lt. Shrank in West Side Story. The shrink in Psycho. Twilight Zone. Mission Impossible. Gunsmoke. Combat! The Virginian. Kolchak: The Night Stalker. Bonanza. Dr. Kildare. Hawaii 5-0. Black Sheep Squadron. The Rockford Files.
And about a million others.
joe flynn, the bunggling aristocratic CO in Mchale’s Navy.
Vincent D’Onofrio
His portrayal of “Pooh Bear,” the drug dealer in “Salton Sea,” opposite Val Kilmer, still gives me the creeps. I knew a few people in my youth who later spent their lives as criminals–or got dead trying–and they were just as cunning, vicious, and utterly crazy as “Pooh.” (Who lost his nose to cocaine sniffing, let that be a lesson to all Dopers). A fine, fine actor.
Just checked in to see if Jack Elam was mentioned. He was.
Brendan Gleeson
Michael Gambon
Keith Carradine
They’ve all had their share of lead roles, but more often than not play very strong supporting characters.
Ben Johnson
Victor McLaughlin
Ward Bond
Warren Oates
I’m seeing a trend here…
Wallace Shawn
I agree with Hank Azaria, wholeheartedly.
He’s possibly too big now, but Steve Buscemi. I don’t like him in Adam Sandler films (luckily Rob Schneider has largely taken that role), but in movies like Fargo, Big Fish, Reservoir Dogs, etc., I think he’s brilliant.
You speak wisdom. See Ebert’s assessment that “no movie featuring either Harry Dean Stanton or M. Emmet Walsh in a supporting role can be altogether bad,” although he eventually found an exception.
More recently, Mark Strong has been there to make good movies great and mediocre movies a bit better. Bad guys in Stardust, Kick Ass, Sherlock Holmes, and I guess Green Lantern. He plays the villain/antihero in RocknRolla, but he might as well be the star there since the title character is a tool.
Mark Strong also had a great role as the head of the Jordanian intelligence services in Body of Lies.
James Cromwell. Guy can be anything.
He played the lead role on a successful TV show for 10 years! IMO, that kinda DQs him as a “character actor”.
gary oldman, one of few who can switch from cuddly-cute to stark raving mad within 10 frames of film.
Yes! Steve Buscemi was the first name that came to mind when I saw the thread title. I fell in love with him in Ghost World.
Monica Horan, who played Amy on Everybody Loves Raymond, is another favorite. With that character, she helped breathe new life into the show, and the relationship between her and Robert was hilarious and just great. Her facial expressions are awesome, almost like a cartoon character or something. I can’t find it, but the scene where Marie walks in on them having sex and Robert comes out in just his uniform top and Amy has the huge bottoms pulled up to her chest makes me laugh out loud every time I see it. Their wedding dance is also great…
Wedding dance
He’s easily my second. He can do comedy (as Ned Ryerson in Groundhog Day) and drama (as Sammy Jankis in Memento). Very good actor.
Gary Oldman. My favorite actor.
That was the first name I thought of.
He also had the lead role in Steal This Movie. And he was chillingly brilliant one version of The Taking of Pelham 123. He out-Shawed Shaw!