Elfen Lied was superb other adult themed cartoons/anime I loved would be Trigun, Cowboy Bebop, Neon Genesis Evangelion. 

If I had to choose a western cartoon then family Guy or Southpark.

About a year ago i rediscovered a show called Gargoyles. i consider it better than any anime i've seen, and i consider it above Batman the Animated Series.

'Adult' anything normally implies bewbs and such, which are all good and all - maybe favourite prime time cartoon bro?
I believe a good Cartoon however is ageless if done right.. take Batman The Animated Series : I LOVED it when I was 12.. now I LOVE IT EVEN MORE now i'm 31.. quality is quality..although, some exceptions can be made for super dumbed down kids cartoons, but there are more universally appealing cartoons than most people realise :)

oh, and Clone Wars gets my vote - far better at actually doing Star Wars right than that abortion of a trilogy.. and Anakin's getting all Darth Vaderish this season.. sh*t be hitting the fan soon me thinks, yee ah yah! :)

Adult Swim had some cool stuff back in the day... I miss my teen years :(

Beavis & Butthead

King of the Hill

You can make an argument for South Park, The Simpson, Futurama, maybe King of the Hill, or a few other shows but if you think a Seth McFarlane show is the pinnacle of adult animation, you're not an adult (or, at least, not a very intelligent one).

Oh, wait. A 15 year old started a thread on the best in adult animation. Fail.

it's tough to pick between Aqua Teen Hunger Force, Boondocks, and The Venture Brothers, but if I had to pick I'd go ATHF. I just can't get enough of that show, I still watch reruns at least once a day when they're on late at night.

That's a low blow, you might want to think twice about who is the immature one in this thread. If you have to bring age into your argument then you are obviously compensating for something.

Low blow? The only low blow here is the one made to the human race by implying that Seth McFarlane creates funny things.