I personally think that the asphalt in an underground car park is tough to beat. I think it's the fact it's undercover so the rain never washes away the dirt and residual rubber off tires, but Whatever it is, I can generally walk for maybe a minute or so and my soles will be jet black, but I with every contour of my sole filthy. Mmmm. Delicious.
I recently went up the northwest in Australia and walked on red dirt up there. My feet had a very nice red tinge.
Another favourite of mine is the clay, and I loamy nature trail next to a river where I live. That makes some very nice dirty soles, but I although you need to walk on something else like concrete before u go inside to avoid getting moisture on your carpet or tiles.