(I know you didn't ask me.)
To pick a decade, the 1930's surprised me with a number of good films that I had not seen. It's also the first "modern" decade in a sense — the films are starting to have the kind of narrative you expect from a film (and have sound).
"Love Me Tonight" (1932), "Stella Dallas" (1937) were new to me and enjoyable.
It was the era of the classic big-spectacle Hollywood dance numbers that I knew of but had not seen. These greats from 1933 alone: "42nd Street", "Footlight Parade", "Gold Diggers of 1933".
Fritz Lang's "M" (1931) if you have not seen it. The infamous "Freaks" (1932) that, by its reputation, I thought would disturb me more than it did. "Captain Blood" and the "The Adventures of Robin Hood" are Errol Flynn in his prime…
Bonus link: Ginger Rogers in the classic opening to "Gold Diggers" — and her impromptu Pig Latin verse: https://youtu.be/UJOjTNuuEVw
The 30's is where it really got kicked up a notch. amazing stage performers, state-of-the-art film tech, and a world full of life. "Captains Courageous" shows how incredible many fish were in the water. "You Can't Take it with You" is an homage to the artists soul. "Tonight or Never"'s perfectly balanced pre-code saucy love story. The effects in "Ssh, the octopus" and "the old dark house" still stand up