*points to Pixar films in general* Allow me to quote Roger Ebert on Ratatoullie:
Roger Ebert said:
This is clearly one of the year's best films. Every time an animated film is successful, you have to read all over again about how animation isn't 'just for children' but 'for the whole family,' and 'even for adults going on their own.' No kidding!
(Full review
here)
Batman The Animated Series was also very well done. Sure, the animation strikes me as somewhat lacking these days, but it
excelled in the characterization department, providing the introduction of Harley Quinn and what has essentially become the
true story of Dr. Freeze (as most subsequent takes on the character have used that story as their base). On a similar vein, it's worth checking out the flicks Batman: Mask of the Phantasm and Batman Beyond: The Return of the Joker (For the latter, the uncut version is
highly reccomended). Granted though, these all harp on comic adaptations, so may not be the best examples.
And I know you said western animation, but all the same I say Monster's worth a mention, if for no other reason than it makes for a very good example of a dramatic adult-oriented animated series