Probably mud, although I find getting my feet that dirty to be annoying. Good grass is second, and a nice tile/marble floor (think hotel lobby or mall) is probably third. Although, we had this fun concrete in my childhood back yard with small smooth river stones in it, and that was really nice as well. Kind of a foot massage as you walk.
For me the whole "thing" of going barefoot is feeling the different surfaces on my soles. I love it and also find it massively sensual, if not sexual. I guess however for me the best is when I go on a long walk of 12-15kms. The combination of concrete and bitumen along with grass and sometimes beach sand is the absolute best. I love looking down and seeing my barefeet touching all the different surfaces. At the end of the walk I also like how my soles feel. Like kinda alive and slightly tender from the walk even although they are like leather!
My soles are not like leather, yet. I enjoy most of the different sensations rough gravel and little bitty stones are still challenging. I think being 71 years old makes the toughening process longer. I have about 15 months of going barefoot as much as possible. Wet grass and mud are the best.
It took me three years of being completely barefoot to get my bare feet tough and like leather. But you will get there the more you go barefoot on cement and black top and gravel your feet will get super tough it takes time and I know you will get there in time barefoot Larry I believe in you .
I like the feeling of grass and coldness in grocery store. Every once and awhile when I get mail from mailbox and walk on our street I get a tingling feeling down there in a private place. Feels good and just wow. I like mud and I’ve walked in the snow just to feel it and it’s cold but refreshing
I prefer cement over asphalt in summer. Cement doesn't get to the scalding temperature as asphalt does.