The obvious choice, mentioned in that previous thread: Ricardo Montlban in STII: The Wrath of Khan.
“From hell’s heart I stab at thee. For hate’s sake, my last breath I spit at thee…”
I’m gratified that the first response to this thread named Al Pacino (and yet he was so subdued way back in the first Godfather movie. Today he’d come out of the bathroom and take a flamethrower to Solozzo while quoting the St. Crispin’s day speech from Henry II).
Anyway, my favorite actor going over the top was Gene Wilder in “Young Frankenstein.” He could chew up the scenery just by darting his eyes around.
Anybody see Wilder in “The Little Prince”?
Albert Finney and Tom Courtenay in The Dresser. Finney is a Shakespearean actor who’s on the edge of a nervous breakdown; Courtenay is his long-suffering, maintenance-alcoholic personal assistant. The movie was originally a play, and between the two of them, I can’t believe they didn’t reduce the original theater to a shambles. But it works: I love this movie with a passion.
Scent of a Woman is surely Pacino’s hammiest performance. He’s definitely one of those actors who needs to be watched carefully.
Jack Nicholson is another prime chunk of ham in most of his movies: Terms of Endearment, Wolf, Batman and The Shining come to mind. He’s always good in this sort of movie, it’s just that the rest of the movie tends to become nonsensical and lame around him.
zeldar mentioned Olivier: in most of his modern roles I think he’s horrible, with the zany accents and over-acting, e.g. Marathon Man.
I hate to mention something so trivial, but I’m remembering this episode of Three’s Company where John Ritter accidentally mixed medicine with alcohol and started telling complete lies to everyone and then dancing around like a fool. It was actually quite funny, but I’d say it qualifies as scenery chewing. (Among Three’s Company fans, and I am frankly not one, this is evidently a favorite episode).
I agree with the following:
Gary Oldman - The Professional (great movie)
Alan Richman - Robin Hood**
And I’ll also add:
Alec Baldwin - Glengarry Glen Ross
He is only in the movie for about 5 minutes, but he absolutely steals it!
“Third place is…You’re fired!!!”
Jack Nicholson in pretty much everything but especially as the Joker in Batman.
And while we’re on comic book adaptations I think an honorable mention needs to go to J.K. Simmons who played J. Jonah Jameson in Spideraman.
I was grinning from ear to ear through every scene he was in.
And I guess you can’t mention that movie without putting Defoe on the list as well. His scenes gnawing on the frame of that mirror are classics.
The entire cast of The Insider. (Seriously, did anyone in that film not have at least one yelling speech?)
Peter Finch in Network.
Any non-Shakespeare Kenneth Branagh film.
Faye Dunaway in Mommie Dearest.
Alan Rickman in Robin Hood. For sure 
The reason I have Lethal Weapon IV in my DVD collection is because of a rant Chris Rock and Joe Peschi get into about cel phones.
Both Peter O’Toole and Richard Harris are up there in the pantheon of all-time scenery-chewers, bless 'em, but they were lucky enough to each find a role that suited their talents perfectly: O’Toole as the scenery-chewing Alan Swann in MY FAVORITE YEAR (“I’m not an actor–I’m a movie star!”), and Harris as English Bob in UNFORGIVEN (“You’re all a bunch of savages, with no laws, and no morals!”).
It’s been ages and ages since I saw Brando in THE MISSOURI BREAKS, but as I recall a fair amount of scenery got devoured in that movie as well.
I’m surprised that Charleton Heston hasn’t been mentioned yet.
“PEOPLE! Soylent Green is PEOPLE!”
“Damn you all to HEEELLLLLLLLL!!!”
Ed O’Neill dined on the scenery a lot during the last seasons of MWC.
I was about to cite Nathan Lane in THE BIRDCAGE in particular.
I am amazed at how buff James Marsters is considering all the scenery he’s consumed as Spike on BUFFY.
However, all three men can underact beautifully and subtly when needed.
Would you count Richard E Grant in ‘Withnail and I’? Or possibly the charming man who played Uncle Monty! Heck, just about anyone in the film except Paul McGann, who can be rather OTT himself when necessary!
I doubt it’s possible to overplay the part of Jasmeson.
Bette Midler. Ouch.
Another vote for Peter O’Toole. Especially after you rent The Ruling Class. yow