I would live in the code geass world just to kill Lelouch and say to his face "Not so great now, *****!"
Claymore - just to kill Raki so he can shut the hell up for good.
Kamen no Maid Guy world would be awesome if I could have him as my maid. The most powerful maid in the world is under my control.
Law of Ueki world would be insane if I get to choose what kind of power I want. Forget changing garbage into trees, changing garbage into food is better. No need to go to restaurants when I have the entire earth's waste as my meal. And screw using it for fighting, there's a reason why I have arms and legs.
Also does manga count? Then I would have to say Mx0 world. Finally a school with magic that isn't crap. I would learn some uber spell and just terrorize the school and be the school bully.