He's part of the original 40, and since then his kit falls pretty much lackluster to almost every other champion in the game and he's not played very much - but what stays true is that he was the original "protagonist" of the League of Legends story and played the original role of the big muscle manly guy who was all about justice, honor, and fighting for his country. Many people have taken a liking to this guy for his fairly comical style of play which usually involves running into everything and not caring, builds such as DFGaren have risen overtime as one of the most popular troll builds (sad that DFG is being removed soon, though...)
His sibling-hood with Luxanna and companionship with Jarvan/Xin Zhao are one of the most well known/were one of the first established alliances in the game's lore. It's safe to say that Garen will always be one of the original faces of the game, and one of his most iconic quotes spread through other champions that came after him in the roster. 8)