I have been on an adventure game spree lately and I've bought most of those games that have been mentioned (the ones by lucasarts at least). Fun!
All the old LA adventures are goooood. The only one I don't have is Sam and Max, which I should be getting sometime soon. If the PC versions are too expensive, you can always buy the Mac cd off of ebay for cheap as free and then use ScummVM to play.
The Dig: awesome music, desolate feel, etc etc.
Grim: Shweeet. Great game.
Blade Runner: IDK, I got a ways though then I ran into a scripting bug for my savegame... fun till that point though.
Day of the Tentacle: lol, fun
The Longest Journey: My favorite ever. Words do not adequately describe it. Music was beautiful too. I will play a bit of it again in a minute just because of this post..
Dreamfall: Was not disappointed by this sequel (WHERES NUMBER THREE!? JEEBUS!)
Monkey Island games: MI1 was awesome and so are the rest. Entertaining, and it has pirates. What more could you ask for?