whats the best software for terrain modelling and is ther any good plugin for 3dmax to model terrains?

Photoshop.

All rich here and have 3ds max?

make a plane and brick it 72x72

then use a noise modifier to make it. then you can export it, and import it
in MDL.... much work with 3ds max :)))

Better way height and colourmaps with a paintprogramm.

Gimp or Photoshop

It's the past to have such pain workflow !

Today you can use World Editor terrain manager that have the tools for deformation, painting etc ... in main indie 3D engines.

Well let's hope GED to incorporate in a near future all complete tools to edit terrains.