In our opinion, the best paint brush for cutting in is the Wooster short cut with a tapered edge (meaning that there is a dome shape to the edge of the bristles when you look at the brush edge wise - which is called the filament). We typically use either the Wooster Ultra/Pro Firm Shortcut Angle Sash 2" brush or the Wooster 2" Shortcut Angle Sash brush with Chinex bristles. Both of those have that tapered filament that make it perfect for getting a nice clean line for trim.