I played several of Lucasarts' adventure games recently and I think the one that my view on changed the most was The Dig. I mean, I liked it before, but in playing through the whole thing I really appreciated just how great an experience it really is. They perfectly captured the feeling of exploring an alien world and trying to figure out what happened to the former inhabitants. And it's got one of those soundtracks that's just so absolutely unforgettable.
But even The Dig can't pry me away from Curse of Monkey Island, I'm just way too attached to it, for a lot of reasons. It was one of those games that I played when I was younger that really opened my eyes to what was possible. It was like playing a cartoon! And not like one of those gimmicky Dragon's Lair things either, but a proper game. Yeah, a lot of it's from nostalgia, but the presentation, and humor, and characters really were among the best that Lucasarts ever did, and it generally had puzzles that weren't too difficult to figure out.