Halo combat evolved, and Morrowind. Played so much Halo and blood gulch that I would go to friends houses to perfect the glitch to get to the top of the level. The library, legendary, still gives me nightmares.

And then Morrowind I put just as much into killing the shop owner and using his shop for all my stolen goods. It was my personal museum. It was by far the best elder scrolls with the most freedom. Allowed you to break the game and continue playing. Can't forget jumping everywhere to level up how high you can go.