Ah Aliens vs Predator 2, nealry forgot about that one, a load of fun in single and multiplayer and the single player campaign has some awesome scenes.

Not many people remember Drakan. People are missing out from awesome dragon flying action. Fortunately there is a patch to make the game work with modern system.

All konami, capcom and sega titles. just like steam, specially resident evil series.

The only ones I really crave would be Portal and Portal 2. Most games I just shrug it off when they're DRM-encumbered; "Oh well, plenty of other places to spend my money", but these two have really hurt from the very beginning for being Steam, and have just kept on hurting ever since.

Silent Hill 2 , 3 and 4

All of them.

Konami are just jerks so they would laugh at "DRM free" and refuse their games on GOG so it would take a miracle to see their games on GOG let alone steam

Capcom...they're a combination of asshole/idiot they way they handled SF5 gives me doubts, but the RE remakes on PC make me think otherwise. RE remakes will probably make their way on GOG but SF? Nope

Sega aren't anywheres near as much as jerks as capcom and konami but they're flavor is "lovable idiot" they occasionaly do a damn good job but they miss out on a lot of good things. I could see sonic games going DRM free along with older COH. If we see shenmue 3 on PC it will be a wait for shenmue. But having shenmue 1 and 2 on GOG would really help the dreamcast era games on GOG. Which I am a sucker for since they are easy to run and are cheap and are usually good.

Too many to list, but here's a few that come to mind.

The Cave would be top on my list.
I would also like to try Deathspank, that's one Ron Gilbert game I've never played.

Maniac Mansion (Commodore 64 version, if possible.)

Some Pac-Man games, the one with the glitch chasing you looks like fun. :)

Civilization series. I've played only the first two, so it would be nice to try the others.

Diablo series. I'd love to try Diablo 3 some day, but not with always-on DRM... probably not going to happen, but I'd be happy if we get at least Diablo 1 and 2.

Marvel Ultimate Alliance 1 and 2.

EDIT:
I just had to add Drakkhen, when I switched the old EGA monitor for a VGA, this was the game I was playing and I remember being in awe at those sunset/sunrise gradients, lol. Flawed as it is, this remains one of my favorite games from those days, and I would love to add it to my GOG library.

Age of Mythology.
Rise of Nations.
Any Civ don't have a preference.
Might and Magic X Legacy.
The other Dragon Age games.
Rainbow 6 Vegas...any others...god knows how many there are by now :P
Tom Clancy's Splinter Cell: Chaos Theory, Double Agent, Conviction.
Arma 3 + bring back the older games.
Richard Burns Rally.
All the GTR series.
Lock On.
Romance of theThree Kingdoms..any..
Close Combat Two.
Oblivion, Skyrim.
Hammer and Sickle.

Many more.......

... which is also the answer to "How many push-ups does Chuck Norris do?".

All of them.

Off the top of my head I've been wanting to buy here: Oblivion, Fallout 3, New Vegas, Original Doom 3.