Paris!!!!!!

Definitely the metro area (Minneapolis/St. Paul and suburbs) in Minnesota.  They could have it so the main character can travel out of the metro area and go cow tipping!! :P

I would go along with most people and say London.

Considered by most to be the capital of the world, one of the biggest and most populated places on earth. It has pretty much all the history needed for Rockstar to use. It is a violent urban place with many mixed cultures.
In terms of gangs many of the ones Rockstar have used in games have orginated from there.
Lots of new iconic places to rip off.
If anyone remembers the game The Getaway, that was set in London, that game was imense. So it can be good if done well. I think having the right mixture of people and areas would make the game good. You have some of the poorest places in the world in london and then some of the most expensive places on the planet. It is such a divide that they could return to that rags to riches story, which i prefered to that of GTA4.

But for GTA5 i think they need to take it further then just one city. They could have a mixture of cities. The world is a small place now and getting to places is just a flight away.
I would love it if they did London,Sicilly,Johanesbourg,Sydney,Rio,Mexico City,Las Vegas
Then obviously you can get to different cities via flying. It would have such huge potential and covers pretty much the criminal hubs of the world.

Rome or Milan could also look good. Or maybe something exotic like an African or South American city.

Please let it be London, please let it be London, please let it be London.

I'd like a smaller city like Amsterdam. On a smaller scale Rockstar could make it more interactive, this will improve gameplay and experience. Or adding smaller pieces of other big cities to give it an International feel. Powning a whole city is so 1986. These days crime networks have an International scale.

Frankly, when the game comes out, people will most likely play it no matter where it takes place. GTA has gotten to the level where it has a fanbase which will play it no matter if it took place in Detroit, London, Tokyo or Milwaukee, Wisconsin. When it comes right down to it, what's the bigger draw? The city it takes place in...or the guns, missions and oh I donno...Grand Theft Auto going on?

But this does go back to what I was saying before. Will the series start loosing fans down the road and get 'stale'? I don't think simply moving to a city outside of America is the way to reinvigorate the series if that starts happening. They'd have to change the gameplay and entire structure of the game. And I know for me, I loved the driving around and 'lawlessness' of the game, but the missions and storylines were just inane to me. I didn't care if Tommy or Niko had to fight some biker gangs or go assassinate some trader to some mob bosses honor...its all just busy work to the actual fun part of the game, which is getting more guns to use and cars to drive around the city.

So with all that ranting over, what I'd have to say is, a change of a city is not really what the series needs. Its a rehaul of its gameplay. Less focus on the story and 'realism' and more focus on the fun sandbox aspect.

Barring Montana (it would certainly make for some interesting environments!), I would put the next game in Baltimore. Not only is the city covered in crime and conflict, but it's in Maryland, and not too far from D.C. (which would make for some interesting political drama).

Or London, I dunno.

Jakarta... (^_^)

London. The Daily Mail would explode. Probably.