In my opinion, with some rare exceptions, the best substrate will always be natural Sand with a variaty of corn sizes, ideally from a local river. It looks great and the fish feel and will shoe a lot more natural behaviour.
Especially bottom dwelling and digging fish will appreciate it a lot, as it stimulates them way more than the typical same size sand you can buy in every store could. And in combination with a current it will also arrange itself naturally just like in the stream.