Twilight Struggle
"Twilight Struggle" was the highest-ranked game on BoardGameGeek from December 2010 to January 2016. As of August 2018, the board game occupies a fifth spot. Being released in 2005 it is the oldest board game in the overall top 10 ranking.
Board wargame
The actual subject matter of wargames is broad, and many approaches have been taken over the years towards the goals of simulating wars on a grand or personal scale. Some of the more popular movements constitute established subgenres of their own that most wargamers will recognize.
The oldest of the subgenres, and the one that still retains "iconic" status for board wargaming as a whole, it got its start with the first board wargame, "Tactics" (which, ironically, used a square grid; hexes were a slightly later innovation), and is still used in many wargames today.