Well, any LucasArts point 'n' click adventure game, of course. And, I'll second elTee on Gabriel Knight 1, even if I still haven't finished it... Myst and Riven are musts (Myst 3 and 4 are good too, just not as good as the first two). And Riven is much better than Myst even. The Journeyman Project series definitely musts too. And Broken Sword 1 and Humongous games (even if they are for kids!). I'm sure there are others that I have that I love but can't think of right now :P
It's Enrico Palazzo!