should definitely be on GOG. And [url=http://www.youtube.com/watch?v=YKLCXbk9nbI]The Dark Eye, a very underrated Edgar Allan Poe inspired claymation adventure game. On the subject of claymation,
The Neverhood, obviously. Other titles I can think of are
Heart of Darkness,
Dark Seed (and, to an extent,
Dark Seed II),
Silent Hill 2 (at least the PC version of this one should be old enough to be on GOG, and it's definitely a classic, I think) and
Blood Omen: Legacy of Kain. To name a few.
[EDIT] I don't know how much of a *BLASPHEMY* is this, but I'd also like to see the Star Wars: Jedi Knight series being released on GOG. I seriously have no clue as to what the "gaming community"'s thoughts about those games are, but I personally liked the first ones and had a great time playing demos of them in my Win98 days.
[EDIT 2] added gameplay videos whenever possible, as per tfishell's request.