I mostly use the default brushes for webcomics, illustrations and art in general
For example:
I use G-Pen for lineart (and sketching)

After the base color is done, for soft shadows & lights I use soft airbrush

For the solid shadows, I use Diluted Watercolor Brush

Finally for highlights, I use Darker Penecil Brush
For some sketchier artwork, or in specific, the Love Quotes webcomic (Webtoon - Tapas)

I use for sketching / lineart, this Real Pencil

For the grayscale, I use Diluted Watercolor Brush

However, for my Webtoon Contest entry
I'm using for sketching, Lineart and coloring this "Nip With Tips" Pencil (I don't know how to translate this from Spanish, but just in case is "Plumilla con puntas")
Then of course, for effects, ambiance, clouds, etc I just go full on the Asset Store.
Well, my only recommendation would be to just download everything that caughts your attention at the Store (Even more, pay attention since certain brushes are free for a limited period of time before being for pay), it doesn't matter if you're not gonna use it right away, you'll find an use for it later or just to play around.