Judging by the time a game has been on my hard disk it is Operation Flashpoint which is on my disk for almost 7 years (uninterrupted) and I've been using it freqently for these 7 years. It is the most open game to modding I know of apart from Half Life maybe.
Then there is Combat Mission, Falcon 4 (although I never really got into playing it) because of its dynamic campaign.
Edit: I just remembered that Operation Flashpoint was released in 2001 so it does not count as a 20th century game, but well whatever...
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