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
Right now I'd say Tokyo Story (1953) is the best film I've ever seen.
I watched it because it's on every list of best films, so expectations going in were high. It's not overrated. I don't cry from movies but I did when watching this one. Very subtle and relatable.
Edit: Since we're here, "The Fall" (2006) and "City of God" (2002) are some of my other favorites.
(not the OP)