It's easy for me to pick the top 2 because my only experience with Norma Shearer before seeing The Divorcee & A Free Soul was The Women. The early pre-code films are such a radical departure from what I had watched and read about Shearer that they couldn't help but blow me a way. The overt sexuality and carefree attitudes towards fidelity and decorum are eye opening for most viewers unaccustomed to pre-code films. Even if the Academy Award she won for The Divorcee is reflective of different criteria then, the character and the performance are fantastic. The scene in A Free Soul when she brazenly lays down on the chaise lounge and offers herself to Clark Gable's gangster character Ace Wilfong (Top 10 greatest character name) is worth every penny for admission! The Women is a classic film through and through and Shearer is very good in the somewhat thankless character of the wronged wife. The dressing room scene with Shearer & Crawford is a fantastic battle of wills, strength vs strength as both actresses/characters shift from power to weakness as the scene plays out. The stories from the set between her & Joan Crawford are also priceless.
I probably overweight the value of Shearer's pre-code work by ranking Riptide & Private Lives above Barrett's Of Wimpole & Marie Antoinette and I can't argue that both may be more mature performances, but I like what I like. Maybe it's Robert Mongomery that breaks the tie, I don't know (but I bet it's the first time he's been a tiebreaker). The casual banter between he & Shearer is entertaining, if light as a feather. For whatever reason, Tyrone Power just doesn't look right in those period costumes in Marie Antoinette, although Robert Morley is wonderful as King Louis XVI.
Let us Be Gay is an odd one because the normally totally put together Shearer appears unrecognizable in the opening scenes as the dowdy, neglected wife of a philandering husband. She more than redeems her beauty and captivating presence in the later scenes, all the while trying to stay out of the scene-chewing way of Marie Dressler. I like this movie, but it's only the conclusion that leaves me less than thrilled.
I couldn't neglect Shearer's silent work, so I've included both Lady of the Night & her last silent Lady of Chance as representative of the fine work she did before sound came in. Check out the Norma Shearer Bio for a more compete overview.