If I had to make a short list of supporting parts that entertain me more than everything else in the movie combined (so it's got to be really entertaining) I just may have to put Raymond Walburn at the top for Riding High. If you've never seen it, it's a Bing Crosby vehicle directed by Frank Capra on his way down Hollywood's "What have you done for me, lately" ladder. Bing's got a horse but not the money to get it into the proper races and Ray Walburn is Professor Pettigrew, a bumbling half-wit con man who's trying to help out with the cause. Problem is, he's inept. So inept he literally cons himself out of money at one point in the movie ("Bilked! By my own chicanery!"). He's amazing and should've been nominated for Best Supporting Actor but wasn't. Of course, another short-lister, George Sanders won that very year for All About Eve so no real complaints here.
Or how about Elsa Lanchester in The Big Clock? With but a few minutes of screen time she steals the whole show as the flighty, judgemental and needy painter, enlisted to identify the killer (falsely accused Ray Milland while really it's her real-life hubby, Charles Laughton).
Or Noel Coward in Bunny Lake is Missing?
Or Barry Fitzgerald in The Catered Affair?
Or Thelma Ritter in anything?
Or Bert Lahr in The Wizard of Oz?
Or Melvyn Douglas in Mr. Blandings Builds his Dream House?
Or Nina Foch in An American in Paris?
Or Glenda Farrell in Mystery of the Wax Museum?
Or Ernest Thesiger in Bride of Frankenstein?
Or Madeline Kahn in Paper Moon? Or Young Frankenstein?
And speaking of Young Frankenstein, how about Cloris Leachman, Marty Feldman and Kenneth Mars? Or, hell, Gene Hackman?
Or Gene Wilder in Bonnie and Clyde?
Or Richard Pryor in Silver Streak?
Or Dudley Moore in Foul Play?
Or Jeffrey Jones in Amadeus?
Or Elaine May in Small Time Crooks?
Oh hell, there's so many to list and so little time. Give me some more time and I could knock off the drama list just as easily. One spoiler for that one: Agnes Moorehead in The Magnificent Ambersons? Probably at the top.