The first in Sid Meier's long running series, Civilization represents a milestone in the strategy game genre and set the table for a lot of fundamentals that would be replicated by dozens of other games going forward. It takes a lot of the good bits from titles like Empire, Populous, SimCity and Pirates among others and synthesizes them into the foundational 4X title several years before the term was even coined. It certainly wasn't perfect, with certain aspects of gameplay not being as fleshed out as others, but what was there was more than competent enough for 1991 and a fantastic launchpad for bigger and better titles in the coming years.
As a side note, I want to also praise its tutorial components because very few games up to this point bothered to explain game systems and mechanics to players inside the game itself. This was often a memory limitation that resulted in very thick instruction manuals, but Civilization is one of the earliest examples I can think of that provides context menu tooltips to explain parts of the interface which was its own little revolution worth praising.
As a side note, I want to also praise its tutorial components because very few games up to this point bothered to explain game systems and mechanics to players inside the game itself. This was often a memory limitation that resulted in very thick instruction manuals, but Civilization is one of the earliest examples I can think of that provides context menu tooltips to explain parts of the interface which was its own little revolution worth praising.