My Favourite Brushes
I mention my brushes often but I don't always show what they look like. I wrote a section a while ago on how to purchase brushes if you are learning to paint; this is a more in-depth discussion about the specific brushes I recommend for intermediate painters.
There are many shapes and bristle types out there—I found a good online summary here. Contrary to what you might think, you really don't need very many brushes. Once you figure out what sizes you like, you will probably find yourself just using a few brushes over and over again. I'm a fan of synthetic bristles whenever possible, as they are usually cheaper and perform as well or nearly as well as their less animal-friendly counterparts.
Angle Shader
Synthetic/Bristle Mixture
By far my favorite, I use this brush for every single painting I do—often for 90% of the work. The angle brush is essentially just a flat brush cut on an angle. You can use it flat side down for wide flat strokes, or turn it 90 degrees and use it edge on for tight corners. It's very similar to a flat brush, but less common. I get the Princeton Best series and I usually order 5 at a time since they're hard to find in stores. These are very robust brushes, but after a few moths of heavy use with acrylics they start to get fuzzy and the tips spilt. Here's a three month old brush next to a brand new brush:
Round, Medium and Small
Synthetic Mongoose
The classic paintbrush: if you can only afford one, get a medium sized round brush. Look for one that tapers nicely to a point and retains its shape after it gets wet. I use the medium (size 6) brush for most of my fine detail brush, and the small (size 0) brush only for very small detail such as eyelashes, whiskers, and signing the painting at the end. Don't make the mistake of using too small a brush, beginners tend to go for the small brushes but sometimes a larger brush is better for keeping you on track. A small brush can just exhaust you before you get your painting finished.
Big Filbert
White Nylon Synthetic
I use this 1" filbert brush for underpaintings, skies, and areas where I need a lot of color at once, and a brush big enough to spread it around well before it dries. I also like to use the filbert to make puffy clouds, the rounded tip makes nice little semicircles when tapped on the canvas. The brush in the link above is not the same brand I use, but it is a similar type and quality. This brush has lasted about a year and will be ready for replacement soon. As is common with old brushes that have seen a lot of time in the paintwater jar, the ferrule became loose from the repeated swelling and shrinking of the wooden handle. I re-attached the ferrule to the handle with a few wraps of masking tape.
Rake Brush (aka Comb)
Gold Taklon Synthetic
This is a specialty brush that I don't use very often, but is excellent for specific applications where I need to make groups of the tiniest lines possible. Its double layer of bristles is great for painting fur, wood grain, and fine grass. Rake brushes, like angle shaders, are difficult to find so I buy them in bulk. My favorite is the Royal Langnickel with the comfort grip and plastic handle.
Stipple Brush
Golden Taklon Synthetic
I actually made this stipple brush out of an old round brush, but you can buy them in most hobby stores. I use the stipple brush for polishing out brush strokes when I am adding transparent layers and want a smooth, diffuse appearance.
Big Flat Brush
Golden Taklon Synthetic
I occasionally use a large flat brush for applying protective varnish and sometimes for painting skies in very big paintings. I recommend the Blick Mega Brush series, which are intended for large paintings and murals and are excellent quality for the price.