I’d visit the DC Comics Universe (herein known as the DCU). Okay, there are certain places I’d steer clear of – Gotham City probably isn’t the best tourist destination in the world, as you’re probably likely to end up being serial killered by a masked psycho – but it would be cool to go to, say, Metropolis and meet Superman.
Of course, the question didn’t ask when in the history of the fictional world you’d want to go. That’s important. Do you go to their present, or do you get plonked down in the middle of the plotlines you’ve already read about? cos I’d want the latter. The DCU has a bad habit of blowing up cities, and it would be best not to be in one.
At the same time, it gives you a way in to all the cool stuff none of the characters would let civilians get to. Okay, it wouldn’t be a good idea to mention to Batman that you know he’s really Bruce Wayne, and that he should give you a Batmobile or else, but you know, you could tip them off about a few things…
It would also be easier to get a Daily Planet t-shirt than it is in the UK, so that’s another bonus.