Not including World of Warcraft (which I have several thousand hours in), it would have to be Crusader Kings 2 (which I have over 500 hours in), Fire Emblem 7, Fire Emblem Sacred Stones, and Pokemon... Blue? Pokemon Sapphire? Pokemon Silver? I'm not sure which of those first-3-generation pokemon games I had the most time in but I had a lot of hours in all three of those games!