Easy
Fallout 1 & 2

1.Seven Kingdoms
2.Gothic 2
3.Seven Kingdoms 2
4.Warlords battlecry 3
5.Fallout 2

1. Fallout 1 & 2 (easily my favourite games of all time)
2. Giants: Citizen Kabuto ( I always loved this game)
3. Duke Nukem 3D
4. Sacrifice
5. Beyond Good & Evil

1 Fallout 2
2 Crusader
3 Freespace 2
4 Far cry
5 Giants

Freespace 2
Fallout
Descent
Duke Nukem 3D
Unreal Tournament
Although my list seems odd since most of those I own the original disc and don't have the GOG version for one reason or another.

Fallout series (not including Tactics)

1. Screamer
2. Duke Nukem 3D
3. Rise Of The Triad
4. Septerra Core
5. Patrician 3

This is awesome guys. I really like how everyone has such a different list. Why must you guys make it so hard to choose? :)
I'm suprised the Oddoworld games haven't showed up. I really enjoyed both of them, with their strange humor and interesting puzzles.

Well, some of these i don't own off of GOG. But here's in no particular order
Sacred Gold (Good Diablo clone)
Warlords Battlecry 3 (RPG/RTS hybrid)
Fallout 2 (RPG)
Fallout (RPG)
Tallout Tactics (Squad-based tactical RPG.)

SACRIFICE
MDK 2
Cannon Fodder
Hostile Waters: Antaeus Rising
Descent 1+2
Jagged Alliance 2: Unfinished Business

MDK2
The Guild Gold
Sacred Gold
Gothic 2 Gold
Patrician 3
I haven't had a chance to play my other gifted games yet, therefore, this list is subject to change...

I might soon get MDK 2, but I just got The Guild Gold today. Gotta make time for all these games!

1. Fallout
2. MDK
3. Operation Flashpoint
4. Gothic (I'm sure Gothic 2 as well, when I get to it)
5. UT99
6. The Guild Gold
7. Seven Kingdoms: AA
8. Sacrifice
9. Arx Fatalis
and probably more, when I get to them..

I think the whole GoG catalogue has actually been suggest to you if you combine all these lists. Sure helps narrow it down. :)

Jagged Alliance series
Cannon Fodder