A hard brush I love sketching with.
An aliased brush with a wide range of opacity I enjoy using to sketch my art.

The left set of lines is moving the pen fast across the canvas. The middle set of lines is slowly moving the pen down to show the range of opacity with pressure in a single stroke. The right scribble is making a gradient in one overlapping stroke.
Something about this brush lets me feel like I can mess up and it still is fine, which is something I need during sketching. I like keeping it at zero stabilization so I can sketch faster, but feel free to turn it up. Play around with opacity & density too!

Here's an example of the brush in action, slight erasing, no stablization. I don't always shade in my sketches, but this brush makes it fun.

If you're like me, you can use it to do sketching, line art, and coloring. The example above was made only using this brush. It acts like a marker you can layer on the background color, I like it.
Hope you find this brush helpful! :)