Ok PRG fans i got a question for you!

Well i want a good RPG that will last me a while. I want a huge world i can explore and i want to be able to make descions that dont have to effect the story but some how effect the game. Also i would like to be able to fool around if i dont want to do any missions :DI was think of the following....Fallout 3 Fable 2 Oblivion OTHERS??