EarthBound. Because it took me two years (yes, TWO FUCKING YEARS!) to finish that game, since I am brazilian and couldn't read in english when I was 9, so it basically forced me to learn a new language. As a 9 year old. By the time I was done with it two years later I could read a book fully in english and understand every part of the story.

Of course, I didn't finish the game in a single playthrough. Many times I gave up on it, just to return to it eager to make the story move forward and see if I would be able to defeat Giygas, the evil alien from the future. And I was NOT ready for Giygas. I almost crapped my pants when I first met him (it was at night and alone in my bedroom), and that part alone took me over two months until my dumbass figure out what I was supposed to do. And yes, it was two months of dying to Giygas again and again.

But the journey towards the end was awesome too. And I'm pretty sure it's the only game on the planet where you reunite a couple of black and white sesames, ride on the Beatles' Yellow Submarine, uses poison snakes as offense items, finds a guy who lost his shirt in gamble over a solitaire game (seriously), eat stuff from the trash, walk around on your jammies, order pizzas and fights abstract arts, street signs and huge piles of vomit.