Probably the best "Myst clone" of its time, it was one of the only games released by GTE Entertainment, and thus a planned sequel was never able to be produced despite solid sales. I believe it's still being sold in its original glory on 4 CD's through Barracuda Software, though it was a while since I last checked.
2. The Journeyman Project 2: Buried in Time
While the third game in the series is arguably better (see #4), Buried in Time still holds up surprisingly well in this day and age if you can forgive the lo-res background (which were amazing at the time).
3. The 7th Guest
Some of the best writing and devilish puzzles in a computer game coupled with the fact that the FMV actors weren't atrocious made this one the creepiest, most enjoyable games of the early 90's. Too bad the series decided to shoot itself in the foot with the sequel, The 11th Hour.
4. The Journeyman Project 3: Legacy of Time
Great use of the 360 degree view technology at the time, it also tied in well with the other games in the series but could stand alone just as well.
5. Schizm: Mysterious Journey
The simple fact that you're utterly alone in this strange world, walking around aimlessly, solving some of the craziest puzzles there are (trigonometry, really?), really puts you in a mindset other adventure games rarely do. The excellent ambient score and hi-res colorful backgrounds helped of course.
6. The Crystal Key
This games single handedly showed that adventure games were alive and well at the turn of the millennium.
7. Dark Fall: The Journal
The unique story and setting stay with you long after you've finished the game. That's saying a lot for any work of entertainment.
8. Frankenstein: Through the Eyes of the Monster
Tim Curry. Enough said.
A common theme, but an excellent execution even if the end game left something to be desired.
10. Beyond Atlantis
The Atlantis series always felt discombobulated and all over the place. This is the only one in the series that felt somewhat cohesive and flowed well from world to world. Take it as a separate game and forget about the others in the series.