I ignored adventuregames in my first list but here's some comments;
Darkseed 1-2
Ancient point & click adventure in which you play a guy who recently got a house in which he starts to experience horrible nightmares and headaches. Soon enough he understands that he have an alien embryo growing in his head. It had impressive graphics at it's time with artwork of Giger. It was a very buggy game. In the second game you play the same character, who now is on the edge of insanity.
Elvira 1-2
A blend between adventure and RPG, with brutally difficult puzzles that almost always kill you in gruesome ways. In the first game you explore a castle, in the second a movie studio. One fun detail with part two is that you use regular objects, like floppy disks and fire extinguishers, to assemble spells. Very difficult, even with a walkthrough.
I have no mouth and I must scream
A "hit-you-in-the-gut" story about some questionable characters summoned by a big computer for a game. Highly controversial, one of the characters you play is a nazi doctor, another is a rape victim. Each have to face their past. Quite an unique game, I do not know any game that allowed itself to become this dark.
Phantasmagoria 1-2
Gore adventuregames by Sierra, with lots of body fluids thrown around in FMV sequences using real actors. In the first game you play an author who move into a house with her boyfriend where things soon begins to go wrong. Highly controversial in it's time due to a rape scene which is quite tame by todays standard. In the second game you play a guy who at first seem to be a very normal person, but as the time goes on more and more is revealed about his true self. Quite a thrilling story that makes you wonder what the truth behind all weird things happening are.
Sanitarium
Another point-and-click adventure that starts weird and gets weirder. Maps consists of locations like an asylum, a circus, an ancient aztek pyramid etc, and how it all is connected isn't revealed until the very end. Another adventuregame playable for it's plot.
Scratches
Like I said above, MYST style adventure in a spooky house. Beautifully rendered and very realistic setting that makes you feel there. Managed to scare the crap out of me twice. Deeply atmospheric.
Waxworks
Spiritual sequel to Elvira. You inherit a waxworks where it quickly is revealed that the 5 stages in the waxworks are portals through time and space. Each place is a story of it's own, from a zombiefilled cemetary, to an ancient egyptian pyramid. Like Elvira it's a blend between adventure and rpg that can be quite difficult, even with a walkthrough.