I recently thought of this (during down time) while talking to a friend who's relatively new to the anime scene. He hasn't seen that many titles yet, but is almost irrasionaly enthusiastic about those that he's already seen. Considers them the best. Without seeing at least Evangelion or Akira (amongst others), titles which I consider to be among the ultimate classics, I don't consider him that knowledgable on anime. Enthusiastic yes, but not that into it yet.
When I mean classics, I mean anime titles that are to the art form known as anime comparable to any Shakespearean classic in English literature. Now English lit will take you seriously if you don't at least know some Shakespeare. Therefore I wouldn't even include Golden Boy, as funny and wacky as it is, but not that much a classic as FLCL for example. Try do discern between something funny, wacky, soppy and something GOOD, or at least a must or memorable title.
Would like to see members' comments on their own classics to fill up on my own checklist. But firts my own:
Evangelion (especially the movies)
FLCL
Akira
Any Gibli, 'specially Mononoke Hime and Spirited Away
At least one of any Macross (my fav was Zero) and Gundam
Hellsing
Vampire Hunter D (both movies, but at least Blood Lust)
For some reason I dried up...uhm, will get back when Bleach and Naruto finishes (another DBZ series). Therefore please post your nominations.