Okay, I'm not talking about "OMG what's on Adult Swim this week". I'm talking about if you could choose one anime that really is a must-know for anime fans, what would it be? For example, Naruto is pretty popular right now, but years from now are people still going to think it's important, have crazy debates over it, et cetera? There are definitely series out there that every serious anime fan should at least know of and probably see some of, at least I think there are.
In general, I'd say anyone really into anime should know about Evangelion. Given that, no matter what your opinion of it may be, it really set a standard for a lot of things, introduced two of the most prominent sex-appeal characters anime has (and really, even though I'm female I know a lot of the appeal in anime is the sexy factor), introduced a storyline that is still used as a comparison to so many other series, and has an ending that fans still bicker about. Even if you're really a fan of newer stuff, I'd say Eva is an essential tidbit of knowledge.
There are definite subsets, too-- like, I'd say if you want a really definitive shoujo piece, grab yourself a stack of just about anything CLAMP and call me in the morning. And it can also be argued that "true fans" should know a little about classics like Astro Boy and masterpieces like Studio Ghibli's works. What are your thoughts, community?

Because you mentioned Naruto, which should be one that is NOT DEFINITIVE, and SURELY SHOULD HAVE BEEN BURNED IN HELL 10 TIMES OVER, it renders this thread useless, next to a stupid plug.

but I agree that if there is one anime that all people must watch, and must know.

It would be GITS, Ghost in The Shell, which together with classic title Akira, spawned endless Hollywood reincarnation and inspired movies from Matrix trilogy to many others.
It is state of the art, an anime before its time, throughly imagined in a future so intense and so surreal, it makes you feel like you are in an animated orchestra of artistic excellence, suspense, in a world of espionage, in a world of pure science finction, so much so, that sense just gets thrown aside, as a world of indulgence in the technological estima just takes you over.

It is the one definitive title I would recommend, all season 1 and 2 and movies with it.

Initial D. If it weren't for that anime, Toyota wouldn't have sold so many AE86's. That anime changed the way I viewed at cars and racing and popularized drifting

I was noting Naruto as one of those DIE, damn YOU, DIE anime that people only care about for a short time. As an example of what I was NOT referring to.
I didn't even think of GitS, it just slipped my mind I guess (how did it do that?! silly, get back in there *stuffstuffstuff*)

Initial D was the shit, but only because Tuners were popular in America after the great Jap import tuner wave, I don't even think an AE86 was worthy of its so called fame.
Surely, if you have the cash to mod, go get a rare NSX already, now that is teh classic.

on topic however, I would also vote a definitive anime in drama, Air and Air the Movie, being my choice.
Why? because it made me cry like a bitch towards the end, and it still does.
One of the best anime ever, and still is.

Oh yeah, I forgot to mention The Melancholy of Haruhi Suzumiya. Air was realy depressing for my first watch, after watching it again I didn't feel as teary-eyed as before, but I still rank it as one of the best and most depressing animes ever. Kanon was pretty much just as depressing.

Actually I read something today that makes me feel a bit sounder in my decision on Eva... according to Newtype Japan, Evangelion is still the 3rd most popular series among its readers, and Rei still ranks in the top females.

Evangelion is still one of the corner stones in the anime world.

As for Rei...well...it's not hard to see why she's still the best. You don't need words.

Read more... (click to view)

Evangelion is indeed one of the corner stones in anime. It's downright embarrassing to call yourself an anime fan without at least knowing what this anime is about.

Also, as far as the newer generations go, I believe Death Note is up there with anime's that any anime fan should at least look into.

Death Note? Although I'm a rabid fangirl, I don't know how much of a lasting impact it has. If I start seeing more things like it, then I may change my stance. Albeit the controversies that have popped up concerning it are indeed noteworthy, now that I think about it.

I think as far as "cornerstone anime" goes you could probably rule out all "aired every week, never going to end, light entertainment" type shows like, Naruto, Bleach, whatever. Don't get me wrong, they have their place, but they really couldn't be classed as "must sees".

NGE and GitS would both be on the must watch list. Definitely the first "Ghost" movie, and possibly even the second one as well. I'm surprised no-one's mentioned Cowboy Bebop yet as it's one that's referred back to time and again as a classic. Would also venture to put forward Martian Successor Nadesico. Though it's not so well known over here, it was still ranking in the top 3 anime of all time back in it's home country, just behind NGE in fact. If it's slipped since then I don't know, but it was a fantastic mix of storyline, character development and light humor. Plus you didn't need to be on something illegal to understand the ending.

Every anime fan should have seen a few Ghibli films, particularly the earlier work. They're classics, though the earlier films were probably more gritty than what's been offered of late. Take "Grave of the Fireflys" for example.

As to what might make it into a list like this in 10 years time... who can say? Probably SHnY, maybe Gurren Lagann.