Leonard Maltin's 100 Must-See Films of the 20th Century

Published as a preface to his 2000 Movie Guide, and probably several times since. The films are listed chronologically.

(My opinion: Maltin is a lover of classic movies, so it's no surprise that his list is heavily skewed toward pre-1980 films. While there's really not a single bad film on this list, glossing over the final twenty years seems like a gross oversight. Lists are personal, though, so it's useless to argue.)