What software to plan/design a map?

I am making an open-world game which will mostly be comprised of one giant city and have a general idea in my head for the layout but think it would be better to pre-plan the design rather than just trying to build it as I go along.
Please note I am not talking about terrain creation tools like World Machine. I am talking about something which I can help assist me design a well planned map with the general layout of my world. For example I want to split the city into several “districts” or “zones”. An underground subway. A large rural area outside the city. What I want to be able to do is plan my masterpiece before I build it.

Pen and paper is pretty cool, actually. You can also use 2D programs like Illustrator, Photoshop, or maybe AutoCAD. (Or Inkscape, for open source)
Other than that, any tool that lets you drop blocks in to show the bodies of houses and stuff is alright.
Could be 3ds Max or Maya (if those are your tools) or could be an empty “gray body” level you use in Unreal Editor itself. (That’s right: you can use a separate level without gameplay to just test out placement!)