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While I think for most who are asking for surrealistic films, it is usually enough to point them in the direction of the odd or the whimsical. But as Flicker pointed out, it is generally a little more than just that, and for my two cents, films that are just generally strange don't quite cut it for me. For a film to really work as a piece of surrealism, as far as I'm concern, it has to commit some kind of violence of rapture of the relatable or the mundane. And a lot of films that are simply strange for strange sake don't really do this, as they don't establish any reality to disrupt in the first place. Or the surrealistic intrusions are much too lazy for them to cause a viewer to doubt there senses as they are watching (I call this brand 'Just Add Zebra Surrealism', in that it seems to think simply throwing in something that doesn't belong is enough). This is why I think whoever suggested Apocalypse Now as a contender is right in considering that a part of the genre. It builds its sense of reality by hemming fairly close to how a war film feels. And yet, when you start introducing lieutenants ordering their troops to surf during an air raid, and a bloated Marlon Brando speaking great wisdom that doesn't make any sense, it puts the viewer in the position of having to decide how much of what they see can be trusted. Surrealism, when it is effective, should cause some amount of disruption of passive viewing. In a lot of ways, surrealistic artists kind of gaslight their audience.
There have been a couple of films here, that while I think are definitely surrealistic, don't at first glance actually qualify for the definition I provided above. Eraserhead, as an example, never technically gives us any sense that the world we are watching has anything whatsoever to do with our own. And in that way its grotesqueries and non-sequiter scenes seem like they fit in such a world, and therefore, are less likely to cause that fundamental level of confusion in those watching. But, while Eraserheads world is clearly a fiction, Lynch employs so many pregnant pauses and drawn out scenes he creates his disorienting effects here. Much too much credit is given to deformed babies and ladies in radiators as the source of Eraserhead's discombobulation, but the heart of the films surrealism is actually more to be found living in the sparse sound design which seems all too familiar to the lives of the lonely, or the long lingering gazes at elevator doors that take forever to open. This is where he allows some feeling of realism to seep into the dreamscape of his film. Real life is what seems out of place here.
It is by this simliar token that I also feel obligated to bring some amount of criticism to the most celebrated of all surrealists, Jodorowsky. I think by definition he has to apply, but personally, I think he's a pretty piss poor surrealist, in that his entire worlds are competely warped by his imagination. There is no basic reality to pervert. This doesn't mean he's not a great filmmaker. He is. But his power lies mostly in his incredible strengths as an imagist, which while surreal in nature, don't lend to an overall sense of surrealism. Or at least not a very good one.
Most surrealistic films I would recommend have already been listed (Marienbad and Meshes of the Afternoon being pretty strong contenders). Also, I don't know of L'Age D'Or has been mentioned, but it should be (even though it isn't a personal favorite of mine). Also Bunuel > Dali by a country mile.
If I think of anything off the beaten track, I will list them here. So far the only one that is coming to mind right now though, and I wouldn'tn even call it a purely surreal film (even if it slowly degrades into something that becomes harder and harder to comprehend) is the little seen horror movie Eyes of Fire. But there are generally only really bad copies of it available, which might put some off due to the terrible image resolution.