My favorite tank sizes are 15g (20x12x12) and 50g (36x16x16).
20g longs (36x12x12) also work well. I've grown to like panoramic aquarium layouts.
I don't have a favorite substrate. Still working on what works best for me.
My favorite plants to use in aquascaping? That is such a loaded question... I like:
Bolbitus, Anubias, Ammanias, Rotalas, Ludwigias, Didiplis, Hottonias, Hemianthus, Micranthemums, Bacopas, Blyxas, mosses, Heterantheras, Limnophilas... mostly, which is almost everything.
Carlos