Sand of course. Or better, mud!
But it doesn't really matter all that much for most species. Sand probably makes the most difference with carnivorous species that sift sand while foraging for insect larvae and the like. Whiptail cats for example love sand, and will bury themselves right into it sometimes. At the other extremes, the pure herbivores like Panaque don't care, since what they want is wood and plant material. Your common or garden plecs will be somewhere between these two extremes. They'll use sand if it's there, but if using sand messes up the aquarium, don't worry about it.
Cheers, Neale