The Eyeliner Brush
An eyeliner brush is extremely versatile because you can use it to apply a precise line, or to soften and blend for a smoky eye. The key is to find a brush with a flat head and stiff bristles (the width shouldn't be wider than the tip of your index finger). An angled brush will be the most universal and allow for control whether you're using liquid, gel, or powder liner.
Make Up For Ever Angled Eyeliner Brush, $24; makeup forever.com.