As folk horror, A FIELD IN ENGLAND is technically provocative, if not strictly innovative, and I think it serves as an antecedent to something like THE WICKER MAN, as it explores expansive versus limited worldviews. And it does that through the lenses of philosophy, religion, and magic.
The Wicker Man (1973)
I find it to be smart and literate, and I think it gives people a lot to chew on and discuss both in terms of what it presents narratively and visually, and what it proposes philosophically. It's filled with symbolism and foreshadowing, and it doesn't rely on the usual tricks of jump scares or gore to destabilize the viewer.