The Myst series is one of my all-time favorites. I even suffered through Myst and Riven on PSX with the PSX mouse(!) before I had my own PC. I will never get what people are talking about when they claim their puzzles were illogical or arbitrary. See Discworld or Shadowgate for FAQ-necessary games. The Myst games could all be finished just by being observant and using your brain.
I love the adventure genre. The Longest Journey is a masterpiece. Gabriel Knight seems pretty good but I never finished The Beast Within. Sanitarium was oh so fun. Grim Fandango was okay but I hated the control scheme so I never finished it. There are still so many adventure games I haven't tried and certainly intend to someday (like the whole Space Quest series, Sam&Max and DotT, Bad Mojo, Reah, etc.). There are some god-awful games lurking around that I've had the misfortune of trying: The Crystal Key, Cydonia, Broken Sword. Obsidian and The Neverhood go for big bucks on ebay I think or else I'd have bought those two already.
What's that Mac-only Myst-clone that takes place in/on a gigantic guitar? Looks cool.
Go here for a really good adventure game site (but take the reviews with a grain of salt): http://justadventure.com/
