Pool filter sand. Lots of us use it. It's dense enough that it doesn't cloud up, and doesn't get into filters (unless fish spit it in the intakes like my bastard fish).
You also don't have to vac it because detritus sits on top and doesn't get down in the sand. And if you have diggers like I do, you don't even have to stir it up as they move it around constantly.
A no substrate tank is sterile. A thin substrate can increase BB population by 10 folds thereby degrading detritus quickly. A bare bottom tank is actually harder to look clean because detritus degrades slowly and is hard to hide. The substrate doesn't need to be thick, no more than 1 inch as only the top 1/4 to 1/2 inch is aerobic. I prefer 3 to 4 mm dolomite gravel, which provides buffer to the water yet not too fine to get sucked into the filters.