A brush with purpose
Any oil painter will go through many brushes of varying type, material, and size. I have tried so many. Researching recommendations and reviews can only go so far. You need to work with them. Live with one for a while and see if it is a good companion for you. Like a good friend, you take care of it for contributing so much to you. You wish it a long life and wonder what you would do if it went missing or got lost.
As a painter of mostly smaller (12×18” or smaller) works, my most used are as follows:
For underpainting, hog bristles do the aggressive scrubbing in of the paint where the action is like that of three dimensional sculpting. Pushing in around the shadows and mopping the lighter tones around. Filberts are the type I use. 10 to14 size.
Softer synthetics work well in the next stages of blocking in color, when working in lean thin layers. Filberts and flats are what I use mostly. A good soft filbert can be used at different angles to vary the thickness of stroke. Flats are great in defining the rectangles for architectural subjects. Angulars are great, as they can give a pinpoint edge. Mine range from mostly 6-10.
For later details which are applied thicker, a small bright type brush can apply a small amount of paint with a firm emphasis. For small fine lines and details, a fine sable watercolor brush is indispensable. I have a #1 which has lasted many years.
I know this will change gradually. I always like making new friends.