Alot of these have been said already but I'd still like to share my thoughts.
-Diablo 1 (scared the shit out of me and my friend when we used to play late nights in summer)
-Ultima online (my first and last mmo)
-Age of empires (played the day it came out and was addicted for months)
-Sim Ant
-Chrono Trigger (fav RPG)
-Stronghold (not really old but still good)
-Nightmare 3d (old doom ripoff type shooter, loved the music)
-Heroes of might and magic 3
-Age of wonders (love this game probably my fav turn based strategy )
-X-COM UFO (My fav turn based strategy)
-Starcraft (played the shit out of this one till got bored and played td's for awhile after)
-Warcraft 2 (Loved it)
-Counter-strike (My most played game, started playing in '99 when it was in beta and still a mod for hl. quit around the time steam came out and forced you to dl steam to play. Love steam now but back in the day I saw it as the enemy.)