MGM vs. Warner Bros.
The main difference from the Classic Film era and today is the existence of the studio system then vs. now, particularly since (at that time) they owned their own theaters, such that audiences identified with the studio brand names in much the same way that we identified with the "big three" TV channels in recent years. I don't know about you, but I don't really notice (or care) what studio makes which films today, except Pixar and maybe Disney (though this is difficult given the many brands within their family).
Of course, the most obvious result of the studio system and what made their films so identifiable, then vs. now, was due to the fact that most of the major actors/actresses were signed to long term contracts with a particular studio. Therefore, one can easily identify a film as being a Warner Bros. product because Bette Davis is starring in it. However, there were exceptions, and some great films resulted, when these actors/actresses were loaned out (even traded like chattel) by the studio moguls.
Realizing that the studio system's voting policies corrupted some of the results, particularly in the early years, MGM dominated Warner Bros. when it came to Academy Award Best Picture winners (8 to 2) through 1959. FYI, I didn't count Gone With the Wind (1939) in MGM's total. However, and after reviewing two books which I recently acquired (The MGM Story & The Warner Bros. Story), I have to say that it's difficult to decide which studio produced the best movies in the 30's, 40's and 50's. Here's why:
MGM vs. Warner Bros.
1930 - The Big House, The Divorcee, Min and Bill vs. The Dawn Patrol
1931 - The Champ, Emma, A Free Soul, Possessed vs. Little Caesar, The Public Enemy, The Maltest Falcon, Five Star Final
1932 - Grand Hotel, Red Dust, Freaks, Smilin' Through, Tarzan, the Ape Man vs. I am a Fugitive from a Chain Gang, One Way Passage, The Mouthpiece
1933 - Dinner for Eight, Bombshell, Queen Christina, Penthouse, Sons of the Desert vs. 42nd Street, Gold Diggers of 1933, Footlight Parade
1934 - The Merry Widow, The Thin Man, Manhattan Melodrama, The Barretts of Wimpole Street vs. Jimmy the Gent
1935 - Mutiny on the Bounty, David Copperfield, A Tale of Two Cities, Anna Karenina, A Night at the Opera vs. Bordertown, Captain Blood, Dangerous
1936 - The Great Ziegfeld, Fury, Libeled Lady, San Francisco, Romeo and Juliet, Three Godfathers vs. The Story of Louis Pasteur, The Petrified Forest, Anthony Adverse, The Charge of the Light Brigade
1937 - Captains Courageous, The Good Earth, Topper, Camille, Way Out West, A Day at the Races vs. The Life of Emile Zola, The Prince and the Pauper, Kid Galahad, They Won't Forget, Tovarich
1938 - Boys Town, The Citadel, Three Comrades vs. Jezebel, Angels with Dirty Faces, Four Daughters, The Dawn Patrol, White Banners
1939 - The Women, Ninotchka, Goodbye Mr. Chips, The Wizard of Oz, On Borrowed Time vs. Dark Victory, Dodge City, Juarez, The Old Maid, Invisible Stripes, The Roaring Twenties
MGM's dominance seems to turn in the latter half of this century, IMO. In the next decade, with Humphrey Bogart and Bette Davis, I think Warner Bros. starts to dominate the 40's:
1940 - Pride and Prejudice, The Philadelphia Story, The Mortal Storm, The Shop Around the Corner vs. They Drive By Night, Knute Rockne-All American, The Sea Hawk, The Letter, All This and Heaven Too, Dr. Ehrlich's Magic Bullet
1941 - Blossoms in the Dust, Woman of the Year vs. High Sierra, Meet John Doe, Sergeant York, The Maltese Falcon, One Foot in Heaven
1942 - Mrs. Miniver, Random Harvest vs. The Man Who Came to Dinner, Kings Row, Yankee Doodle Dandy, Gentleman Jim, Now Voyager
1943 - The Human Comedy, Madame Curie, Cabin in the Sky vs. Casablanca, Watch on the Rhine, Edge of Darkness
1944 - Meet Me in St. Louis, Gaslight, National Velvet vs. Arsenic and Old Lace, Mr. Skeffinton
1945 - Anchors Aweigh, The Picture of Dorain Gray, They Were Expendable, Our Vines Have Tender Grapes vs. To Have and Have Not, Mildred Pierce, The Corn is Green
1946 - The Yearling, The Postman Always Rings Twice vs. The Big Sleep, Humoresque
1947 - ??? vs. Life With Father, Dark Passage
1948 - Force of Evil, The Search, State of the Union, Easter Parade, Command Decision vs. The Treasure of the Sierra Madre, Johnny Belinda, Key Largo
1949 - Battleground, On the Town, Madame Bovary, Adam's Rib, Intruder in the Dust, The Stratton Story, The Secret Garden vs. White Heat
And finally, here are the 50's. Continuing their trend in the latter half of the 40's, MGM held it's own and continued this throughout the 50's, especially when it came to producing (dominant) Oscar Best Picture Winners:
1950 - Father of the Bride, King Solomon's Mines, The Asphalt Jungle vs. The Flame and the Arrow?
1951 - An American in Paris, Quo Vadis vs. Strangers on a Train, A Streetcar Named Desire
1952 - Singin' in the Rain, Pat and Mike, Scaramouche, The Bad and the Beautiful, The Naked Spur, Ivanhoe vs. ???
1953 - Julius Caesar, Mogambo? vs. House of Wax
1954 - Seven Bridges for Seven Brothers, Executive Suite vs. Dial M for Murder, A Star is Born (Judy Garland/James Mason version), The High and the Mighty
1955 - Blackboard Jungle, Love Me or Leave Me, Guys and Dolls vs. Bad Day at Black Rock, East of Eden, Mister Roberts, Rebel Without a Cause, The Court-Martial of Billy Mitchell
1956 - Lust for Life, The Catered Affair, Forbidden Planet, Somebody Up There Likes Me, Edge of the City vs. The Searchers, Baby Doll, Moby Dick, Giant
1957 - Jailhouse Rock? vs. A Face in the Crowd, Sayonara, The Pajama Game, The Spirit of St. Louis
1958 - Gigi, Cat on a Hot Tin Roof vs. Auntie Mame, Indiscreet, Damn Yankees!
1959 - Ben-Hur, North by Northwest vs. The Nun's Story, Rio Bravo