Another childhood classic. Probably THE most mod-able Battlefield title in the entire franchise, and certainly THE best. Battlefield: Bad Company is second, in my opinion, due to its humorous story, and introduction of (major) destructible environment to the franchise.
But for this game, just the base game alone is not enough, we need the entire collection of every "official" DLCs this game has to offer for the complete experience. The amount of mods this game has is incredible, to say the least. Battlefield 2: Nations at War is one example of a "major" one, and if memory serves, it's still very much alive. Ditto for Project Reality as well, if I'm correct. And that's not mentioning the countless smaller mods (mostly maps and QoL improvements) for this game.
I only had the chance to play a pirated version of the title back when I was using my old PC (running Windows 7), but alas, even that's lost media now. My only hope for a DRM-free experience with this game is either a second-hand physical copy of the Complete Edition, which is not easy to come by in this day and age, or GOG.com. Can only hope for the best.