What I most want to see on GoG is not a specific title, but rather a catalog of titles. What I would most like to see on GoG is the rest of the Sierra/Dynamix/etc catalog, especially the Sierra titles. Most of these are one or two-off titles, such as Codename: Iceman, the EcoQuest games, Willy Beamish, the two Laura Bow games, the two Conquest games, and many many more. It's a shame that these games are not on here. I'd be willing to pay a hefty premium to get the entire Sierra/Dynamix/etc catalog placed on GoG.
After that catalog, I'd like the whole Lucasarts and Westwood (sp?) catalogs here as well, though I want the Sierra catalog first.
To me, these companies and their catalogs are the whole reason that a place like GoG is even in existence in the first place. DRM-free and downloadable as a separate executable independent of the 'platform' and thus able to be backed up, as well as cheap and affordable, is what I love about the titles on GoG, and I'd love to see more love for these companies.
(P.S.- I do appreciate them branching out some into newer titles as well, though, since it makes them DRM-free.)