Oh, I almost forgot - Hidden and Dangerous and Hidden and Dangerous 2 are also great. They're very similar to Arma (but released before Arma afaik, and have smaller maps). Pretty solid, but I do feel that the controls have not aged well at all (for example, they use the "end" key to aim down sights by default). They are overall pretty enjoyable, but to be honest, nostalgia aside, you aren't missing a ton if you don't play them. There are a lot of rough edges, such as grenades not killing enemies that are close because a chair leg was in the way.

If you're gonna play any of the games I recommend, I would say play Vietcong and give it a real shot. The game starts slow but by the 3rd mission it gets intense