The Longest Journey is IMHO the best Adventure game I have ever played. Great dialogues, graphics, plot, puzzles. Being mainly an adveture game player for years I have played a lot of them. But Longest Journey is one of the few games that when I finished it left me content and happy. I have enjoyed very much this game.
Grim Fandango was nice too. For it’s time. And the 7th Guest was a hit the time it was published.
I would suggest Post Mortem too for the outstanding atmosphere. They could have developed this game more regarding the plot. It had great potentials. Still the atmosphere is beyond imagination.