Many Star Wars fans consider Knights of the Old Republic to be the greatest Star Wars game ever made, and that's largely due to its story. The gameplay fit its time—utilizing round-based combat, a tiered skill tree for Force powers, and an alignment system that tracks dialogue and character choices to determine if you skew towards the light or dark side of the Force—but we'd wager that people remember the profound story more than the way they played the game.
Set thousands of years prior to the rise and fall of Darth Vader, Knights of the Old Republic chronicles the journey of Revan, a character poised between either destroying the Sith or becoming their next Dark Lord. The choices made throughout the game result in either a light or dark side ending, and depending on what the player chooses, they may lose several companions along the way. Sure, the lightsaber duels were cool and all, but what really brought the story home for fans was the exploration of the Sith (the likes of which we hadn't seen before), as well as uncovering Star Wars lore.
Even though Knights of the Old Republic technically isn't canon, aspects of the story have since trickled over into the main continuity. Most recently, we saw Hammerhead Cruisers debut in the Star Wars Rebels animated series, which were later used in Gareth Edwards' Rogue One: A Star Wars Story—and that's only the beginning of what Knights of the Old Republic has offered the Star Wars franchise.