Haha, in fact I’ve been asked this question a number of times this month; I should do a nice FAQ or something. I’ll elaborate a little this time and may actually use it as basis for a FAQ. :)
I’m a bit lazy. I have tons of brushes but I don’t use them! My default brush is the default Photoshop’s Hard Edge Round brush with minor modifications:
(Yep, lots of brushes…)
In the Transfer panel I set the Control for the Opacity Jitter as “Pen Pressure”. Sometimes I control it manually (hotkeys 1-0), but that’s rare. I also turn on the Size Jitter when I feel like. You can open this panel by pressing F5.
I use this brush for basic color and shape work and finer details.
But wait, there is more!
For the painterly brushstrokes I’ve been using a lines brush from Tatitati Brushes Pack. Rendering everything by hand isn’t very clever and takes a lot of time. I tried lots of brushes before to improve my speed, but I only felt comfortable with these brushes. That’s a first!
It’s not over yet!
In the past I’ve used PaintTool SAI a lot. Sadly I started to move away from it for the lack of 64bit release versions, but I still use it some times. I essentially use the brushes from this lovely Brushes Pack. My favourite is the Flat brush:
The fun thing about SAI is adjusting the brushes on the fly. You can tweak Blending, Persistence and Dilution and even Pressure Sensitivity in the same window.
By using this flat brush I accidentally trained myself for the “painterly” style. Because this brush “breaks” in steep angles(1) I had to mind the strokes direction and work with shorter ones(2) sometimes. It helped me to develop an eye to flow and volume (and I’m still working on it!), what is essential for evident brushstrokes, because one misplaced stroke can flatten a solid.
That’s it. I hope it answers some of your questions.