Operation Flashpoint was such a breakthrough. Most realistic war sim set on huge open world islands, similar in scope to PUBG maps. You can drive every vehicle, fly aircraft, command your own squad of soldiers. It had realistic "heavy" controls and ballistics. One of the most advanced enemy AI to this day probably (back then I didn't even realize they can't see you in vegetation. Concept was too novel in 2001).
On top of that it had a great story, felt like being in an actual conflict during Cold War. There are some unforgettable missions like the one where your squad us wiped out by invading Russians and you have to crawl your way back to extraction through various forests. That mission is early in the game presented a big difficulty spike that filtered a lot of players. There was a mission were you escape from captivity and have no map or compass and use stars for navigation. And final mission set after the war where former soldiers travel on the islands (now full of civilian NPCs) to catch up was also memorable. It's like you're an actual war veteran now. Great atmosphere.
There was also very cool multiplayer which I played for a while. Arma (from same devs) superseded this game and improved in most areas, but it didn't feel the same. Operation Flashpoint was an addicting and unique game.