Shooter/action department:
--musthave
Doom (1,2,3)
Quake (1,2,3, maybe 4)
Half-Life (1+addons,2+episodes)
Return to castle Wolfenstein
Unreal
Unreal Tournament ('99,'04)
Serious Sam (First and Second encounters)
S.T.A.L.K.E.R. (remark: it also fits into RPG department)
Deus Ex: Human Revolution
Portal 1,2
Team Fortress 2
Beyond Good & Evil
Diablo 2
Thief 1,2,3
Dishonored
--shouldtry
Deus Ex
Painkiller
Serious Sam 2,3
Star Trek Voyager: Elite Force
Warhammer 40000: Space Marine
Wolfenstein 3D
Duke Nukem 3D
Aliens vs. Predator 1,2
Overlord, Overlord 2
Diablo
System Shock 2
Bioshock
Strategy/tactics department:
--musthave
Civilization 3,4,5
Starcraft 1,2
Warcraft 3
Warhammer 40000: Dawn of War 1,2 (+ addons)
Total War series
SimCity 3000,4
Caesar III/ Pharaoh / Zeus
Homeworld 1,2
Commandos 1,2
Command&Conquer: Tiberium Wars, Red Alert 2, Red Alert 3
Supreme Commander
Ground Control (1,2)
Majesty: Fantasy Kingdom Sim
Heroes of Might & Magic III
King's Bounty: The Legend, Armored Princess
Jagged Alliance 2
--shouldtry
Master of Orion 2
Heroes of Might&Magic 1,2,4
C&C: Tiberium Dawn, Tiberium Dawn, Red Alert, Generals
Warhammer 40000: Epic Final Liberation, Rites of War
Warhammer: Dark Omen
Settlers series
Earth 21XX series
Age of Wonders series
Galactic Civilizations 2
Star Trek: Away Team
Imperium Galactica Series + Nexus: The Jupiter Incident
X series
RPG department
--musthave
The Elder Scrolls: Morrowind, Oblivion, Skyrim
Fallout: 1, 2, 3, New Vegas
Arcanum
Planescape: Torment
Might&Magic series
Wizardry series
Baldur's Gate series
Divinity Series
The Witcher Series
Icewind Dale
The Temple of Elemental Evil
Dragon Age: Origins
Neverwinter Nights Series
Point and Click department
--musthave
Space Quest Series
King's Quest series
Legend of Kyrandia series
Sam And Max series
Full Throttle
Monkey island Series
Syberia Series
Deponia Series
The Longest Journey series
Goblins series