Brushes are important! You should be actively deciding which brush to use for every part of the painting.
Do you want to use a wet brush or a dry brush? With soft edges or hard edges? Do you want the paint to mix, or blend optically?
Once you've decided what type of brush to use, you should be able to find it quickly with a few different options to chose from. If you have a library of 9999 brushes you'll never find a suitable brush, and end up using the same 5 at the top of your brush panel. Keep your brushes organised.
If you're not using some brushes, either delete them or experiment with them until you learn which situations you should use them. The less brushes the better.
These are the brushes I use at the moment. It's a collection of only high quality brushes, all organised by brush type.
Although I've selected them according to my personal taste, (I generally prefer using hard edge brushes) this is a very complete library.
I recommend you replace your own library with these, and learn to select your brush by the categories I've organised them by. If you have some brushes that you really can't live without - save them separately, and then add them back in to your library after you've loaded mine.
The first category at the top of the brush pack is labelled BASIC. This includes a basic hard and soft brush, as well as about 15 of my favourite brushes. If you want to just use the basic brushes for everything, they will cover all your needs.
All the basic brushes can be found again in their correct categories - but I keep copies at the top together, because you can use < and > keys to quickly scroll between them while you're painting.
The next categories are:
Hard edged (Wet) - Hard edge brushes force you to paint with definite intention. Good for painting silhouettes, hard surfaces like architecture, and for painting details.
Hard edged (Dry) - These are great for doing rough blockouts, and give you some nice texture on the canvas. I like to use these earlier in the painting process.
Painterly (Wet) - These will simulate oil paint or watercolour. Very versatile brushes, you can use these in most situations. Generally I use these during the middle stages of a painting.
Soft Edged (Both Wet and Dry) - Good for blending, painting round surfaces (fabric and skin), soft shadows.
Rough edged (Dry) - Mainly for organic surfaces. For me these are the hardest brushes to learn to use. It's nice to use them to define forms inside a selection or clipping mask, so you can benefit from their nice texture and form sculpting, without getting messy edges. You can use these early on when blocking out a painting to get some nice texture everywhere. Be careful using them towards the end of a painting - they are hard to control!
And some situational brushes at the end:
Texture - Most of these are for painting over existing brushwork, and will just add some texture over the top. Don't overdo them! These are opaque but will blend optically.
Scatter - Also for adding texture. Situational.
Nature - Some various grass and foliage brushes. If I'm going to use these, I will paint over them later with some regular brushes to break up the stamp look.
Pattern - You will probably never use these.