Call of Duty, the very first one.
When it came out, it went besides almost all conventions for not only WWII games but FPS games in general. It depicted realistic scenarios (most of which were lifted from real happenings and operations), it incorporated team play like no other game before it. The AI was very good for its time with enemies flanking and using grenades cleverly instead of simply rushing ahead to stand in the line of fire, it also had some fresh and exciting chase scenes as well as great variation in missions that I had never seen before (from parachuting down alone into a field, driving tanks, escaping in a convertible to getting thrown into Stalingrad frontlines without a weapon). And the thing that really set it apart that all other CoD games after have continued; pacing. This game was just so frantic and different, stress levels actually rose from knowing that the enemies were deadly and you didn't have unlimited ammunition nor did you have an unlimited number of teammates so if one of them died it could make a big difference. The multiplayer was also great fun, with realistic weaponry, limited ammo, fun maps and addictive gameplay in its own kind of naive simplicity.
All in all, this game shocked me, I was used to the shooter where you walk around at your own leisure through corridors with pipes with every odd enemy jumping in front of you shouting boo, finding no cover, simply spraying bullets or melee charging heedlessly. There was no sense of danger, or subsequently, accomplishment to what you did (imo). Call of Duty had its weaknesses for sure but they were rather few and in summary I can honestly say that it is easily top three in my shooter favourite list!
