Best 3-D Modeling Program For BeamNG

There isn't a specific program for BeamNG.
You can use any program that you can use for any other games too.

Blender is usually the most used because:

It's free and opensource.
It has all you need for content creation.
It's awesome.

It may be a bit hard at the beginning to learn (It's 90% hotkeys, people pretty much memorized them).
BeamNG uses Blender too for official stuff (Some cars where made in Blender, all the terrains props were made in Blender aswell).

3DSMax is the paid alternative. It's a professional software. Has more functions than Blender (accessories, nothing critical or must-have).
It costs.

Avoid Sketchup, if used with the default settings (What 95% of the people do), it's just going to screw up your concept of 3D modelling.

If you want to learn 3D Modelling Blender is the best choice.