my answer is my favorite game of all time, The Legend of Zelda. the original, on the NES.
i spent countless hours on the floor of my bedroom with hand-drawn maps strewn about me, trying to find out WHICH ROOM HAD THAT SECRET PASSAGE? I KNOW I WROTE IT DOWN SOMEWHERE!
years later, i set out on a seemingly endless quest to burn every bush, bomb every wall, push every stone. i had to break this game down completely and discover every last secret.
many years later, i still have these maps and hints that i discovered.
i play through the game probably once a year and get absolutely enamored every time. i've played (and beaten) it every way imaginable. play through with only three hearts? yep. get to Gannon without nabbing the sword? double check. play it blindfolded? well, no, that just sounds crazy and i'm totally not buying a blindfold on eBay right now.