Battlefield 2 (and its expansions) was a pinnacle in online gaming. Huge maps, big player counts, a solid progression system and endless mods. I had played many online games before this but this was the first game that truly had me hooked in my early teen years. I would play it for hours and hours a day with friends.
This game needs to have functioning multiplayer for it to be worth adding, though there are clients out there right now that allow you to play MP and it is still a blast.