I honestly have to say somewhere like London or Paris.
There’s more than enough heroes in the US, making her an international hero really makes her stand apart from other heroes. Not to mention the fact that they are cities with incredibly long histories and LOTS of potential for magical/mystical elements.
I also like the idea of her being a nomad with multiple homebases all over the world. Nothing about her really needs to have her tied down to a particular location. When she left the island it was specifically to travel not to find a new home.. Her mission is to "man's world," facilitate global peace. She’s shouldn’t be tied to a particular city in it. Her supporting cast are globetrotters/international and not based in one location. None of her villains are location specific. That is one way that WW is different from most heroes. She is an international hero more than anything.