I'd say:
A couple of ground cloths. One for rural or open, and one for ash waste/moon/super urban.
Roads are really much more important. So roads are a must.
Trees/Vegetation – Depending on whether you are going full alien or more terrestrial vegetation is critical for establishing scale, mass, and starting to provide obstructions for line of site and firing. You can't just use your 15mm or 28mm trees either. They need to be much smaller and should be done as masses of woods to save you set up time.
Hills/outcroppings/masses of rockpiles – again depends on your setting, but directing line of sight, fire lanes etc is easily done with hills and outcroppings. And since nothing is flat, they are a must even in a city.
Structures – Whatever your flavor – gothic or rural or ultra modern, unless you are fighting over landform or other feature, a lot of objectives and scenarios are going to be driven by built up areas of some kind. I prefer cut out felt panels, painted to represent the town proper then place buildings, towers, features on them.
Felt Shapes – I use cut up sections of various colored felt to represent fields, swamp, rough, etc. Its cheap and you can do large sections of it. And it heps break the monotony of the terrestrial green open ground cloth.
Just a few thoughts as I go through what is in my collection and what gets put out every time.
I also would say for the economical gamer in 6mm, if you yearn for the days of the old GW Epic Space Marine carboard buildings, go to the Dropzone Commander website and get the free pdfs of their buildings. They are 10mm, but scaled down they are perfect for 6mm. Print and glue to some foamcore building blanks and you are all set.