Agree that it’s hard to choose one of the paint/paper/brush triad, but I’ll try to do ONE of each:
Paint: DS Moonglow because it’s so versatile (but I’d miss my DS Burnt Sienna!)
Paper: Arches CP–I can live without it because I have less expensive papers, but rather not.
Brush: My Silver Black Velvet Brush #10 because of the great point and ability to hold water.
Ask me again tomorrow and I might give different answers!!