Which terrain editing software would you recommend?

Hello everybody, I am looking for some terrain editing software which will allow me to create high quality terrains.
However I need something with the ability to allow me to flatten and add terrain, something which gives me alot of control over the detail of my terrains. I don’t mind if the program also randomly generates terrain, as long as I can change it with some brushes or a similar function.

If you have tried any of these which would you recommend?
or which combination would you recommend if some work really well together?

Grome 3
World machine 2
Geo control 2 (demo wont work)
Terragen

Thanks for your time.

Edit: I have decided on mud box and world machine 2.

Have you looked at Axis Game Factory (its a Unity tool) it seems to offer a lot of terrain functions with ease of use.