My favorite paintbrush is a NEW paintbrush. A favorite eventually becomes a least favorite as it loses its point. There are many Da Vinci brushes that I really like:
Round Pure Kolinsky Red Sable
CosmoTop Spin Flat Wash
Cosmotop Spin Travel Round
Russian Blue Squirrel
428 Artissimo Paint Brus
I've also been using cheap brushes from Joanne's, found near the decorating paints. These are great for small details when you don't care so much about how much paint is loaded into the fibers. They serve well for accuracy and having a sharp, manageable point.
I love my Neptune wash brush for laying down an even coat of clear water before I begin, or putting in a big loose wash on a large piece of paper.
It's good to have an arsenal of natural fibers (for the purpose of holding a lot of water in the brush for gestural, loose brushwork) and synthetic fibers (to have snap and control).
Also, it's great to have all manner of shapes and sizes, from teeny tiny to massive: from small points, small flats, to large round and large wash brushes.
Small flat brushes are wonderful for architectural details such as windows, and large flats are essential for glazing.
I have some stiff, cheap brushes that I reserve fro gouache or what I call "rough work". like dry brush and scrubbing.
For a short video about Winsor & Newton's Series 7 Brush, click here