I would have to say Vampire: The Masquerade - Bloodlines.  Despite the bugs in it I honestly had more fun playing it than Far Cry, Doom 3, and Half-Life 2.
Far Cry is my second pick mainly because it did almost as much as Half-Life 2, and came out first.  Also for me, Far Cry came out of nowhere, I hadn't heard anything about it and then all of a sudden there it was.  I was anticipating Half-Life 2 since I first heard about it and I started playing it as soon as it was released on steam.  All it was, was Half-Life with physics! and graphics! yay...maybe after the story is completed I'll appreciate it more.  Finally Doom 3 was...meh, Doom. 3!
Onto Bloodlines...Starting last week I have finished this game twice already, and I am preparing for the third run through as a different clan.  The re-playability is amazing, all the NPCs respond differently to different characters/stats and all the lines are voiced over by professionals.  Again, quality/quantity of voice acting is amazing.
edit: adding more...
Bloodlines: The sounds/music from the game really sets the mood as well.  There was one level at a haunted hotel that was really creepy.  It genuinely scared me...The second time I played through the game I actually took off my headphones for parts of the hotel 

After a patch this game could be nearly perfect to me.  There were far too many typos in the subtitles, and the memory leak is annoying if you play for awhile at a time.  Those are the only two things that really stood out to me.
After finishing Far Cry and Bloodlines, I felt very satisfied, like it was worth my time to finish the games.  That feeling just wasn't there with Doom 3 and Half-Life 2.
Hopefully someone comes up with a mod for Half-Life 2 that surpasses CS in popularity so CS can finally be put to bed.