As an aquascaping judge, I can tell you with certainty that there is no one "best" species to use in an aquascape. And if there were, it would stop being the best, because it was over-used.
Each ‘scape needs something a different, and good aquascapers are VERY careful about their choices. Glass cats are not a common aquascaping fish and hard to use well, but they were the perfect fish in the winning IAPLC tank in 2016.
Some things to keep in mind for your choices... decide whether you want a large school of small fish, or a few "statement" fish. If you want a large school, USUALLY it is best to use fish that will school well. More individuals of fewer species usually display better. Decide whether you want the bright sparkle of brightly colored fish, or if a more understated species will allow you scape to shine. Both can work; it depends on the scape.
Some of the biggest mistakes are fish that are so bright and flashy that they take the eye away from the scape and that don't school well. Examples would be red sword tails, Bettas and long finned rosy barbs, the orange Discus forms and goldfish. White or albino fish look terrible in most aquascapes. I've yet to see a tank where these have been used effectively. At best, they don't draw the eye TOO negatively, at worst, you can't take your eyes off them... and not in a good way.
Other bad choices for most tanks are large, fast moving fish. For instance, I LOVE Puntius denisoni. But they don't do anything for an aquascape that is less than 6 feet long and you need a group of at least 6-8 of them.
Some of these are my personal biases, but I think you will see that I am not the only judge who feels this way.