If I've got an internet connection, had high player numbers and the game was modernised, it would have to be Delta Force: Joint Operations. 75 vs 75 on huge maps but with choke points to contest. It would take about 10 minutes before you even saw another enemy on some maps. It was a game way ahead of its time and I wish that Novalogic would make a new one (anyone actually know what happened to them?!).
CS:Source comes a close second. Again, only if I can bring with me the community that used to play on the old 40 player GBUK and Teeside servers.