What does the best possible fill dirt look like?

[WinterDeere]

Clay for all but the top few inches, then organic product mixed into top 6'ish inches. Clay gives stable fill, the organic product gives something for grass or groundcover to take root, and provides some drainage.

For that organic product on top I've used something sold locally as Earthmate, essentially composted sewage sludge and old mulch, which has worked well for me. I order it mixed 60%/40% into regular "topsoil", which around here is code for "clay sifted for rocks". Not as acidic as mushroom soil, and doesn't rob as much nitrogen from your plantings as composted mulch alone.

 

[milkman636]

Clay in my area is mostly gray. The only good quality is that it retains water in the root zone and minimizes the need for watering stuff that I grow. Its horrible for building on because of the amount of shrink and swell it has while it gains and loses moisture. There many types of loams that have varying aounts of other stuff in them, including clay. Each one has it own ratings for different purposes. Here's a link to a rabbit hole if you're interested in learning a bit about the dirt types in your area:Web Soil Survey - Home

 

[Oaktree]

Clay here is OK when it's dry, but gets really mucky & sticky when wet. OK I suppose for a place you won't be walking/driving on, or as a base under better stuff, but not high on my list of desirable fill. I guess a lot depends on what's available where you live.