To achieve the best finish... use a quality brush. The right brushes for the job will also make the work go along easier.
Brushes are available in a variety of styles and sizes. When buying a new brush, first think of what you intend to paint with it, and what kind of paint you will be using oil / latex.
Consider theses 3 factors
As far as brush size you will need several. Painters will have there own preferences in what brush to use for a particular job.
Here are some general recommendations for brush sizes
For exterior siding, floors 4", 3 1⁄2", and 3"
For trim work, cabinets, shelves door panels cutting in walls& ceilings 3", 21⁄2", and 2"
For window sash and spindles 2", 1 1⁄2", and 1"
Shown is here a good starter set of brushes. Note that there is a set for oil base and another set for latex
Material the brush (bristles) Be sure the brush you choose is designed for the type of paint you will be using oil or latex Brushes at the store will be labeled such as.... "For All Paint" or "For Oil Only"
China Bristle is for oil only Water base latex paint will deform the bristles.
Synthetic Bristles such as Nylon, Nylon Blends, Chinex are usually labeled for all paints.
You may consider a brush a brush designed for all paint to be a combination brush, but we recommend using each brush either for latex (water based) or oil / alkyd based. In other words you will have a set of brushes for latex and a set for oil. Now you may intend to use just one type of paint, in this case you will need only one set of brushes.
Note The reason we recommend using brushes made for all paints in only one type of paint (latex or oil) is because there may be residue left in the brushes after they are cleaned. There will especially be residue from paint thinner in a brush after cleaning it from oil paint. This may effect the performance of the brush when it is used in latex. It may also effect subsequent cleanings as there may be two bases of paint residue in the brush at the same time.
To choose the style or cut of the brush
Angular sash brushes are best for cutting in.
Square cut brushes can also be used to cut in (but are not particularly designed for cutting in) they are usually thicker and hold more paint than a sash brush of the same size so you will want this style of brush for large areas.