It's hard to go wrong with any of them, really. You should play through the original Zork trilogy. I really liked their mystery titles, especially "Deadline"; I seem to recall that you had a lot more interaction with the other characters in the game (i.e. the suspects). The "Hitchhiker's Guide to the Galaxy" game was pretty good, too; even though I'd already read the books, some of the puzzles were tricky to figure out. I think I still have my "Don't Panic!" button that came in the box with that one.