In 1968, the movie industry instituted a voluntary rating system with films classified as G, M, R, or X. G was general audiences, M meant parental guidance was advised, R restricted admission for under 17s without an adult, and X barred under 17s. M later became PG and X became NC-17. PG-13 was added in 1984.

Many acclaimed films have carried R ratings over the years and are among the most honored in film history.

To determine the best R-rated movies of all time, 24/7 Tempo developed an index using average ratings on IMDb, an online movie database owned by Amazon, and a combination of audience scores and Tomatometer scores on Rotten Tomatoes, an online movie and TV review aggregator, weighting all ratings equally. Only movies with at least 100,000 user votes on IMDb were considered. Cast and director credits are from IMDb.

Nearly all the top films have 90% or higher Rotten Tomatoes ratings. Martin Scorsese and Francis Ford Coppola directed several of the movies on the list. Other famed directors represented include Sidney Lumet, Roman Polanski, Miloš Forman, Steven Spielberg, Quentin Tarantino, and James Cameron. (Here’s a list of all Francis Ford Coppola movies, ranked worst to best.)

Alfred Hitchcock’s 1960 “Psycho” is the oldest, rated retroactively before the system began.

The top R-rated films span many eras and genres, but are accorded legendary status for their cinematic excellence and cultural impact.

Click here for a list of the best R-rated movies ever made