In no particular order of preference :
Dominions 3
Civilization II & IV
Mechwarrior II (The Clans one - need more of it ! since n°3, it is more a shoot'em up than tactical ...)
Alpha Centaury (of course
)
Transport tycoon (the old one under DOS
)
X-Com / UFO (also the first one - still playing it !! Thanx Undertogs !)
Myst serie (well, until IV, I don't have played the V ...)
Fallout Tactics
Silent Storm (EXCELLENT ! Why no mods ? Why no sequel ??? Bouh !)
Jagged Alliance 2 (soon the n°3 : wait & see)
TIE fighter
Unreal Tournament (the firtst of the name !!!)
Unreal
Half Life (the first too)
S.T.A.L.K.E.R.
Combat mission : Beyond Overlord (the most fantastic tactical wargame !!! The 2WW sequels are also very good. No comment on the last one ...)
Master of Orion I & II
Master of Magic (A classic ...)
Heroes of Might & Magic I, II, III & V (not the IV : pleasant, but not in the spirit)
Pokémon Pearl/Diamond (Heu ... Sorry, just PC games, that's it ?
)
Morrowind
General Fantasy
Stars!
Syndicate
Arcanum
Diablo I & II (but especially the 1rst)
Age of rifles (old, but still a ref.)
Stell Panther 2 (the modern warfare - a extremely huge disappointement that Combat Mission: Shock Force is not SP2 w/ the engine of CM ... I want to play the Falklands with the CM system !!!)
Warlods III: Darklords rising (yes ! It was FUN !)
Tekken I, II, II (heu ... sorry again ...
)
Well, that's it for the moment. I'm not at home and it is just from memory, but that's the best way to select the games which really leave its mark on us.