I was not on this forum when this article was originally printed, but I thought it was a good list, if a bit random. There's a few on there I had not seen that I've enjoyed checking out.
The Onion - AV Club | Inventory: 15 Animated Films For Grown-ups
(in no particular order)
1. Akira (1988)
2. Fritz The Cat (1972)
3. The Plague Dogs (1982)
4. Waking Life (2001)
5. Fantastic Planet (1973)
This is my favorite animated film ever, just so weird, summon the beasts of combat!
6. Light Years (1988)
Lesser Laloux, but the I love the mutants
7. South Park: Bigger, Longer & Uncut (1999)
8. Vampires In Havana (1985)
cheap animation but very funny
9. I Married A Strange Person! (1997)
10. Mutant Aliens (2001)
I wish watching a Plympton feature was as fun as watching 15 Plympton shorts, but somehow it just isn't
11. Heavy Metal (1981)
12. Rock & Rule (1983)
13. When The Wind Blows (1986)
Anybody seen this? The comparison to Grave of the Fireflies makes me want to see it, but it's VHS-only availability taunts me
14. Shinbone Alley (1971)
15. The Triplets Of Belleville (2003)
Please add any of your favorite recommendations, especially if it's available on DVD but might be overlooked.