Loom may not exactly be obscure by any standard, but I don't see it being mentioned nearly as much as, say, Day of the Tentacle or Monkey Island. But it was a truly revolutionary way of reimagining the adventure game genre, and in a very early age of point-and-click. No inventory, single mouse click interaction, using spells to interact with the environment...
Of course, you'll want to play the original floppy version to get the full story; the CD-ROM version had its dialogue heavily truncated to fit onto the CD.
What's your pick?
I'm going to say Hokus Pokus Pink. It's not the greatest adventure game ever made, and its puzzles are fairly simplistic if I recall, being aimed at a younger audience. But it's got a certain weird charm to it. It's compellingly bizarre!
The writing is surprisingly good too, and the Pink Panther and his utter cluelessness about what's going on makes for a fun protagonist.