Most of these films were taken from the list in renown film critic M.K. Raghavendra's book 50 Indian Film Classics. These include films in Hindi, Bengali, Tamil, Telugu, Marathi, Urdu, Malayalam, and Kannada.
I also added some myself:
1. Three silent films from Dadasaheb Phalke because he was Indian's first director and the films are a joy to watch/easy to find online.
2. Two Gujarati films, because for some reason Raghavendra included none.
3. The 2010s movies are about the director Dadasaheb Phalke and also P.K. Nair, who was a celebrated Indian film archivist.
4. I added DDLJ because I don't think you should watch 50 Indian films and have none of them be DDLJ. :)
See also my list on Indian musicals.