Here are some of my favorite brushes. Most of these are are out-of-the-box brushes that come default with painter. Before you try any of these out though, use the "Brush Tracking..." feature in Preferences (edit menu on PC, Apple icon on Mac I think). Do a few strokes in the box typical of your pressure and speed and it will automatically calibrate Painter's settings to match your drawing style. If you don't do this you may not like the "feel" of a lot of these brushes.
I'll start with the brushes I like to draw with:
I use the round tip pens for inking, but I changed the Minimum Size of the brush to zero so I could have lots of scale variation. If you make these brushes very big you lose a lot of control.
The charcoal is the closest I've found to pencil if I'm touching up a pencil drawing I scanned in.
The Liquid Ink looks great but is a bit hard to control, especially when the brush size is small.
Next up, my favorite cover brushes. These brushes have little or no bleed, so it's easy to get fully saturated color:
I don't use the square pastel often but it's a great brush if you want heavy texture in your strokes. I often use the fade tool (Ctrl+Shift+F) to get lighter strokes since it's at full opacity.
My favorite "painterly" brushes have heavy blending at low pressure and apply color at high pressure:
Custom 2 (settings below) is a very digital-looking painterly brush, but it gives such smooth yet slightly varied results that it can be really appealing when used correctly. This is very similar to the brush Ryan Wood uses and until a couple years ago was one of my most-used brushes.
Custom 3 (settings below) is about halfway between the palette knife and Custom 2. I haven't used this one much lately but it's a great brush (I'd forgotten, but this tutorial reminded me) and I think I'll try to use it more often.
There are two other brushes I occasionally use:
The Digital Watercolors are really nice for initially applying color over drawings. I know Painter has some super-advanced watercolors now, but the old digital watercolors are faster and more approachable. Just remember "Dry Digital Watercolor" in the Layers menu when you're ready to paint on top of them.
One final note: Even the best brushes won't make you a better artist. What brushes can do for you:
1-Speed up your process of applying and mixing color
2-Add visual interest with your brush strokes and layering of strokes
However, I've found that 1 and 2 don't usually go well together. In fact, usually the more interesting a brush looks, the harder it is to paint with and vice-versa. So adjust your expectations accordingly: if you prefer quick and easy-to-use brushes like me, don't be terribly surprised when your final results look like the digital airbrush job on some guy's truck. Or, like me, you can just accept the cheapness of the results and be happy with the extra time you have because of it. But if you really want things to look great, you need a lot of patience.