Hmmm to pick one is like giving a guy a choice between 50 really hot models, and saying "Choose one".
Alright, well not that extreme; however, i cant name one.
Doom Series - Revolutionized gaming as we know it. May not have had the best story line, but it would have to be my hands down greatest game of all time
Half Life Series - Not just 1 and not just 2. I played 2 before i played one. WOW. I would definitely say it revolutionized gaming, but where it lacks in "evolution" it makes up for it in an intense well developed plot line. I would have to say HL2 is the greatest game i have ever played (Although memories of playing Doom with my Grandfather still stay with me :rose
X-Wing and Tie-Fighter - Well you kind of have to like Star Wars to really like this, however, i absolutely loved playing these games and flying all the missions. If you got the books based on it, the plot line was all that more intricate.
Command and Conquer - Well not THE greatest, but its simply a classic. Good times, i remember always trying to afford a fleet of Goliath Tanks
Duke Nukem - Certainly not the greatest game, but as with C&C it is a classic. Lines like "Its time to kick ass and chew bubblegum....I'm all out of bubblegum" will stick with you forever
BF/BF2; CoD series, Brothers in Arms; FEAR; Far Cry; KOTOR Series; Quake - These, while i wouldn't even consider as the greatest games of all time, definitely are top rate games. The stunning graphics and the numerous awards, as well as their plot lines are very nice. Not the greatest but about as close as you come. So essentially while the above games get the gold, these games are definitely the silver games.
And for the runners up, i would definitely have to say Heroes Of Might and Magic series, Age Of Empire/Mythology, Final Fantasy Series, Flight Simulator Series.
I have honestly heard very good things about the original Starcraft, and Warcraft series; however, i cant accurately comment as i haven't played them.
(For some reason i think i missed a game or 2 along the line)
-Kevin