Stalker is top 10 all-time for me. Thoroughly stunned me the first time. The simple techniques AT used to create something otherworldly - utterly masterful. The ride into the zone - transfixing.

Since you liked Mirror, I highly recommend Yuri Norstein's Tale of Tales. 29 minutes. It's on Youtube.

Ozu is my guy. A truly experimental filmmaker in the narrative space with his own cinematic rules. On GP, I love it. His low camera height, the mathematical shot lengths, 360 degree staging/crossing the 180 degree line, the pillow shots, graphic matching/games/patterns, elliptical stroytelling, etc.

His 30s work is very different with lots of editing of shots of short duration, camera movement, and flashy imagery. His later color works might be my favorite period because while they were not comedies they were his funniest works. There's dark Ozu, too (e.g. Tokyo Twilight).

Renoir's innovations is commonplace now. Extensive use of deep staging (not deep focus) and long takes and preference for real locations. Proto-neorealist. Rules of the Game is elevated, rightfully so IMO, by its context of production and release in its geopolitical climate. Without the context, on stylistic merit, it's a film I'd choose to study and analyze if I were teaching film studies.

Off the top, my three favorite 3+ hr films: GF2, Celine and Julie Go Boating, and Jeanne Dielman.