Oh my. Thistle, there are some graphic novels I always recommend to those who think they're still for kids and haven't known of the changes.

Watchmen. Redefined the whole damn comic book genre.
Sandman. Neil Gaiman is a genius, and very dark.
Transmetropolitan. Insane, Cyberpunk, and hilarious - for a given sense of humour.

I also happen to be quite fond of the Punisher under the Max (Marvel Adults) imprint.