I'm gonna nominate this early Gen-X classic,
River's Edge (1986) (My other top pick would be
Fargo, which someone already mentioned. Third place would go to
To Die For, with Nicole Kidman and Matt Dillon in one of his most likeable roles)
This has it all. A dark tale, based loosely on a real incident in California in the early 80's, in which a high schooler murdered a classmate for no real reason whatsoever, and his classmates all made treks to look at the body but didn't report it. The Baby Boomers were confounded by the real news item when it happened; it was the first hint that they had raised apathetic, desensitized monsters and it raised alarm bells.
We apathetic desensitized monsters generally loved the flick, although younger people have reported to me that they find it "corny". I guess you had to be there...
Crispin Glover at his most unhinged and Dennis Hopper (on the comeback trail in 1986 after a lost decade) provide many of the overt laughs, but there was a sinister dark humor in almost everything, from the dialog to the cinematography (which was very low-budget; this was an indie film) This was also one of Keanu Reeves' first films. It was Ione Skye Leitch's debut.
This Tim Burton film is dark enough that nobody in their right mind would call it a comedy, although it is darkly funny enough in overt ways that it can't quite be called serious; it walked a fine line, and it tells a compelling story, rich with detail right down to the flabbergasted Baby Boomer high school teacher, who bored his students to death with claims of how the Baby Boom ended the Vietnam War, and can't wrap his head around this seemingly brain-dead and emotionless new generation. The younger, elementary school Gen-X'ers are portrayed as downright psychotic. It conveys a sense of suburbia cracking up, and things taking a real turn for the worse.
If you caught this at the right time or the right age, you know why I love this film. If you come at it the wrong way, you might just find too amped-up and melodramatic. But I see those characteristics as benefits to the film.