According to the IMDB top 250 list, here's how many made the list by decade, and some of their top rated in each decade.
1920s - 5. The Kid. Metropolis.
1930s - 7. The Wizard of Oz. Gone With the Wind.
1940s - 13. It's a Wonderful Life. Casablanca. Citizen Kane.
1950s - 30. 12 Angry Men. Rear Window. Seven Samurai.
1960s - 20. The Good, the Bad, and the Ugly. Once Upon a Time in the West. Psycho.
1970s - 20. The Godfather. The Godfather II. One Flew Over the Cuckoo's Nest
1980s - 29. Empire Strikes Back. Raiders of the Lost Ark. Back to the Future. The Shining.
1990s - 41. The Shawshank Redemption (Ranked #1 overall on IMDB). Pulp Fiction. Schindler's List. Fight Club. Forrest Gump. Good Fellas.
2000s - 51. The Dark Knight. LOTR: Return of the King. LOTR: Fellowship of the Ring. LOTR: Two Towers. The Pianist. The Departed.
2010s - 34. Inception. Interstellar. Whiplash. Django Unchained. The Dark Knight Rises. The Force Awakens (Probably the newest movie on the list, ranked #78).
My take....There was some talk in the 80s movies thread that the 70s was the superior decade. Despite the 80s having more in the rankings than the 70s, I think the 70's was the better decade by way of having more "largely undisputed" great movies in them. Undisputed as in, people look at you funny if you haven't see them and/or don't think they are any good at all. I mean, I'm not a Star Wars guy, but I've seen them, and can see what all fuss is about for fans of that kind of thing.
Undisputed as in, my wife said she never saw Jaws, so we stopped what we were doing and I popped it in the DVD player right away.
Starting with the 60s undisputed movies, I count 5. Psycho. The Great Escape. Good Bad and the Ugly. Cool Hand Luke, Butch Cassidy.
Undisputed movies of the 70s, I get 10. Godfather (1 and 2). Jaws. Dog Day Afternoon. Cuckoos Nest. Taxi Driver. Star Wars. Deer Hunter. Apocalypse Now. Alien.
Undisputed Movies of the 80s, I only get 7. Empire Strikes Back. Raging Bull. Jedi. Scarface, Terminator, Back to the Future, Die Hard.
Undisputed of the 90s, I say 15. Good Fellas, Silence of the Lambs, T2, Jurassic Park, Pulp Fiction, Forrest Gump, Shawshank, Se7en, Heat. Good Will Hunting. Big Lebowski. Saving Private Ryan. American History X, American Beauty, Fight Club.
The 2000s are real disappointing, IMO. Like, you have to throw in all 3 LOTRs to even make a longer list. And no, I haven't seen them. Otherwise, my undisputed list would include just 5, of the 51 top 250 movies. Gladiator. Requiem for a Dream. Oldboy (Which a lot of people haven't seen because it's Japanese, but you should). The Departed. Dark Knight.
2010s is too new to see what will stand the test of time. I'd say Gone Girl, Wolf of Wall Street, and Whiplash will stand the test of time. Still need to see The Martian and the The Revenant, but they are way too new to judge anyway.
So by decade, I think the 1990's was the best.
1990s
1970s
1980s
2000s
1960s
I'm not going to pretend I've seen anything with Charlie Chaplin in it.
Fun Fact: Titanic is NOT in the top 250 list, despite wining a boat load of awards, and bringing in a ton of money. Did I miss something? I there a reason among the general population that movie isn't held in higher regard? It's fantastic, aside from the predictable ending where, spoiler alert....the boat sinks.