I used to love doing that, watching crazy films out of your box on acid, but now I'm a responsible father
Any film that's not got a linear plot and seems like lots of random stuff happening would be good in my humble opinion.
Naked Lunch - definitely. Helps if you've read the book (as with most book to film conversions) and if you know about William Burroughs life.
Requiem for a Dream - I watched that stone cold sober and came out of the cinema shell shocked, dunno how freaky that'd be on drugs.
Fantasia - yes the Walt Disney film. Might not freak you out but me and my mates found it hillarious on acid.
Jacobs Ladder
Dust Devil - a little known art house film
Baraka - again might not freak you out (like a nature film set to music) but a lovely thing to watch while tripping
Very Bad Things
Apocalypse Now - crazy film when sober, so fun when tripping
2001 - i used to hallucentate when i was a kid and i had the flu, and i saw the last part of this film in one of those states when i was maybe 5 or 6. Needless to say it has made a lasting impression on me!!
Donnie Darko
Cheech and Chong - Up In Smoke
I'm sure there are quite a few more I've seen, but they have slipped my mind at the moment..(nothing to do with intensive drug use in a former life)
Enjoy!!
I.