The Longest Journey is one of the best adventure games ever made. The dialog and characters are second to none, the plot is magnificent, deep and long, the locations are imaginative and awe-inspiring, and the general atmosphere of the game is something you'll want to linger in for weeks to come. This game also deals with some pretty mature concepts, which was/is pretty rare in this genre.
To me, this game marked the end of the era of adventure gaming. This is the last truly awesome adventure game. Fortunately Telltale Games started some kind of mini-renaissance in adventure gaming, so there's still hope. But to top The Longest Journey, well, let's just say I'm not holding my breath.