Deadline

The first non-Zork game Infocom did, which means the parser doesn't understand a lot of abbreviations which most of us take for granted.
However, don't let that stop you: the writing is where the game really shines.
You're a detective who's got excactly 12 hours to prove that the suicide of Mr.Robner was in fact a murder, and prove who did it. You do this by investigating his mansion and questioning the people there. These are some of the most fleshed out NPS's you'll see in a game of this era. They react to a lot of items you show them.
The puzzles are logical and satisfying to solve, but still quite difficult. I found myself using the InvisiClues a lot.
My conclusion: one of the best mysteries Infocom did, if not the best.