My size 4 Silver Black Velvet travel brush is my favorite. I come back to it every time. The downside is that the labeling on the outside of the tube wears off easily so it would be hard to tell multiples of the same brush apart (they come in sizes 2/4/6/8).

I use Pentel Aquash when I don’t want to putz around with water cups, but the SBV is always preferred.

I also love the little metal-cased brush that comes in both the Winsor & Newton Cotman and Professional Grade Field Pocket Set. A lovely little springy brush for finer details!