In no particular order... I love all these games. In terms of emotional impact I'd say OFP, Freespace series, Thief series, and Battlezone series are the most significant for me though.
-Operation Flashpoint: Cold War Crisis (and Resistance and Red Hammer)
-Red Orchestra
-IL-2 Sturmovik (yet to be surpassed a decade later... the mods add considerably to it too)
-Freespace 1 and 2
-Thief series (yes even the third which I loved even though everyone else seemed to not)
-Starcraft 1 and 2
-Battlezone 1 and 2 (so freaking amazing and unique, if you haven't played them... GET THEM NOW, they've aged very well, even the first---I would KILL for a third)---note, I'm NOT referring to the Atari game
-Rainbow Six series (up until Raven Shield, after that it turned to poo)
-Battlefield 2142 (unusual for me... but I really had a blast playing it back in the day and still do occasionally)
-Tie Fighter (incredible at the time... only superseded by Freespace series)
-Combat Mission: Barbarossa to Berlin
-CnC Generals (great stuff back in the day... loved GLA
)
-And for nostalgia's sake... Red Alert, the game that got me into all this
I see so many people mentioning Deus Ex... is it worth playing today or is it too dated except for those who played it when it was released and get the nostalgia factor?
I'll also add that Battlezone 1 is free to download... people still even play it on multiplayer (though the SP is where it's at for me, great storyline and missions and very challenging unlike most of today's games)