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.
The ones that come to mind are probably the old monster movies like Dracula and Frankenstein or some of the early Three Stooges movies. But even then, they are likely not watched en masse except by lovers of genre pieces.