Operation Flashpoint: Cold War Crisis is my gold standard for realistic first person shooters.
Released in 2001, it was capable of doing things most of today's shooters can't or won't do. At a time when most game worlds took the form of small, arena-style maps and indoor settings, Flashpoint took place on whole islands several square kilometers in size, full of forests, valleys, mountains, villages and bases, and it did this with no in-game loading screens, at all. If that alone isn't impressive enough, the engine also simulates ballistics (bullet drop and travel time), sound travel time (a supersonic bullet reaches you before the sound of the rifle that fired it)... even the tides change depending on the hour and time of the year.
This is a game that was modified into a training simulator (VBS1) which is currently in use by several of the world's militaries. The realism and difficulty is such that playing mainstream "war" shooters feels like an amusement park ride in comparison. In this game, every bullet that wizzes by is a potential instant death, being shot in the legs means crawling until you can find a medic (if you can), and you rarely get close enough to be killed by an enemy hand grenade, because often the last thing you see is a dot on the horizon just before they shoot you.
Furthermore, on top of simulating infantry combat in great detail, it also features a huge variety of vehicles (everything from tractors and boats to tanks and fighter jets), all of which can be controlled by the player (or players) and all of which are depicted with a surprisingly high level of detail and realism. It also does an incredible job of putting all these vehicles to work in practice and depicting the complex relationships between infantry, armor and air elements in war (combined arms warfare).
Just about anything NPCs can do, you can do. In the long and varied single player campaign, you start out a lowly private in the U.S. Army, following the orders barked at you by your superiors. Much later on you receive a promotion to Lieutenant and can take charge of up to 11 soldiers. The command system is complex and detailed, and adds a huge tactical element to the game. Want to tell your AI sniper to stay low and target that guy over there but not fire until you give the order? You can. Want to split your squad into two or three teams and search a forest? You can do that. Want to order an NPC to pick you up in a helicopter? You can do that.
As squad leader you can use your initiative and try a wide variety of solutions to achieve your objectives. In stark contrast to most games in which you're led around by the nose and rarely make any real decisions, in Operation Flashpoint you're simply given some objectives to complete and some resources with which to do so, and exactly how you go about it is up to you. Want to go in heavy with a tanks? Go ahead, but you'll lose the element of surprise. Want to sneak in on foot and set up and ambush. Sure, but you better hope the enemy doesn't have a tank escort.
Then there's the fact that any and all of the scenarios in the single player game are also possible in multiplayer. One of my favourite missions involves two groups of guerilla fighters fighting in two seperate locations 3 kilometers apart, while a third group fly an A-10 tank killer and a Blackhawk helicopter in support. Humans lead AI or vice versa, and there can be any combination of infantry, armor and air power in any given mission. The potential for diverse gameplay is huge, from serious and drawn-out cooperative missions to outlandish scenarios. Want to have a player versus player capture the flag? How about a bicycle race? What about a dogfight in WW1-era prop planes? All of that is possible.
And to top that all off, OFP was its developers first ever game.
