I got about 600 comic books, from Prince Valiant to Astérix, from Watchmen to Lanfeust de Troy, from Bone to Modesty Blaise...
For mature comics I´d generally recommend my two favourite comics countries: Belgium & France:
Moebius (absolutely everything, but especially the legendary series "The Incal", which he created together with Jodorowsky)
François Bourgeon (series “Passengers of the Wind”)
LeTendre/Loisel (series ”The Quest for the Time Bird”)
Christin/Mézières (series ”Valérian and Laureline)
Schuiten/Peeters (anything from their brilliant "Cities of the Fantastic" comic books)
Enki Bilal (anything, but especially the “Nikopol” trilogy)
Griffo/Dufaux (series ”Samba Bugatti”)
Lewis Trondheim (anything, especially his hilarious "Dungeon" series)
