These are the games that I can remember as instantly loved by me:

Settlers 2 - my first pc game
C&C Red Alert
Starcraft 1
Stronghold
Dead Island
All Total War games besides Medieval 2 and Rome 2
Cities in Motion 1
Ports of Call Deluxe
Imperial Glory
Dead Space
Railroad Tycoon 3
Dune 2000
Emperor: Battle for Dune
Master of Orion 2
Warhammer: Dawn of War 1 + all the expansions
Warcraft 2 + Warcraft 3 + Frozen Throne

-Sonic 1,2,3 (Mega Drive)
-Cool Spot (Mega Drive)
-Aladdin (Mega drive, by Dave Perry)
-Sonic Adventure (Dreamcast)
-Shenmue 1+2 (Dreamcast)
-King's Quest 7 (PC, the only part I played so far)
-Myst series (PC)
-Atlantis 1+2 (PC)
-Syberia 1+2 (PC)
-Back to the future - the game (PC, great for a fan of the movies like me)

For me, Space Quest and Microsoft Adventure. Much later, Wolfenstein 3D, Doom, X-Com UFO Defense, Crusader: No Remorse, and Grand Theft Auto III.

Out of the Park Baseball
RR Tycoon
Civilization
Pirates
Half-Life 2
Space Hulk - New version
Jedi Knight - Dark Forces
FTL - my favorite new game I have bought on GOG
Heroes of Might and Magic 2
Realms of Arkania
Starflight
Red Baron - (one of the reasons I originally joined GOG.)
Fantasy General

Super Mario Bros. 3