I’ve used a few different brow fillers – both pencil and powder – and have found one I’m currently loving! It’s the Anastasia brand Brow Wiz pencil ($21). It has a crayon on one ends that glides and fills in very naturally and a brush on the opposite end.
I love that the crayon is retractable (so no sharpening) and how smooth in fills in sparse areas. It’s easy to use and comes in 10 shades (I got the ‘ebony’ shade).