thats really sweet!! ;o;!!
I use the default g-pen, chalk, and transparent watercolor brushes a lot! Chalk is what i mainly use for sketches.
At the moment though my absolute favorite brush is Flashito! I'll leave a link in the comments!! This is a hard square brush i like to use on my quick personal art. I like how easy it is to make dynamic line art and the slight color shifts are fun to play with.