I’m wondering what of the oldest films are still watched on a regular basis by a relatively mainstream audience purely for entertainment purposes (as in, not for a film studies class or for the explicit intention of “going through the classics”).

The oldest examples I can think of are Snow White and the Seven Dwarfs (1937) and The Wizard of Oz (1939). I think the fact that they’re both in color and are children’s/family films has helped them age well, even compared to movies several decades younger.