If I'm assuming the role and attributes of a character in the world then FFVII is an easy one. Massively varied, tonnes to do and see, rolling vistas, chocobos to wrangle and breed, Weapons to overcome! The world of Skyward Sword is a close second|
If I'm my soft, fleshy self without the aid of super strength, augmentations or fire flowers then Viva Pinata. Great world, the crazy gardening looks fun as does the animal attracting (ooh err etc).
Cray cray ba nay nay