To be honest I am quite addicted to buying watercolour brushes, and have quite a collection at home. But when it comes to my sketching kit I pare it down to the bare minimum as I found that I work so quickly that I rarely change brushes when I sketch. For many years I used a #6 or #8 round sable travel (pocket)brush – various brands: Da Vinci, Escoda and Rosemary and Co (the one pictured above). I also carried a smaller #4 travel round brush.
But these days, I nearly exclusive use a dagger brush. Series 772, 1/2 inch sable blend brush by Rosemary and Co. (refer to post below for more details).
I also carry a water brush, but only rarely use them. I haven’t yet found a way to get the effects I want with a water brush – the washes are flat and dull. However a water brush is great for fast sketching. I also use it to pre-wet my palette- a drop of water in each pan, and to pre-wet my page is I want to drop in watercolour, especially for skies.