The first few weeks after I bought Skyrim, I would play it for hours on end each and every day, but I eventually lost interest. Same story with Team Fortress 2 and Super Smash Bros Brawl.
Probably my greatest obsession, one which spanned months on end, was with Minecraft. I built so many houses, looted so many caverns, and spent so many hours designing complex strip mining methods it's ridiculous. And this was in Alpha! Even before hunger, leveling, generated structures, enchantments, potions, and bosses, it kept me occupied like nothing else. I don't play much nowadays, but when first discovering the game and getting to know the strategy involved was one of the best gaming experiences of my life.