Hello Popmas,
What is the best terrain editor is hard to say, but there are a few such as T.ED and EarthSculptor are very good and the A.L.E, which is reasonable, in my opinion.
How to importing the terrain stuff into FPSC that's the question.
That it is very complicated task given that will be issues with collision or any collision at all between the characters and terrain.
Furthermore, a drastic drop in frame rate in game screen due to the huge amount of triangles in the scene at once, since the FPSC has no LOD (level of detail) system besides the terrain textures will lose their definition when you get closer of them.
With respect to 3D modeling software, please click the following link and when you get there look for my post where I point out some good 3D modeling software.
http://forum.thegamecreators.com/?m=forum_view&t=196801&b=21
I hope this might clarify a bit your doubts.
Have fun!
Northern