After 25 years painting this small handful of brushes produce most of my work.
Other than a couple of Chinese calligraphy brushes my three main brushes are:
1 inch flat Taklon
1/4 inch flat Taklon
#2 Taklon liner

My favourite brush is the cheap old 1/2" bristle brush available from your local hardware store they produce fantastic expressive lines you just can't match with a neat tidy watercolor brush.

The liner is great for fine delicate lines. The 1" flat is a good all purpose brush, great for washes and, because of the square end, capable of making accurate geometric marks.
For more organic and calligraphic shapes I use either the Chinese brushes or the 1/2" bristle brush. On large paintings I often resort to an old 2" bristle house painting brush. It looks pretty rough but is fantastic on a large sheet of paper!

With water based painting it is very important to have complete control over the amount of water in your brush. I have found the best way to do this is to always have an old towel covering your work table. It's a simple matter then to wipe your brush as you work.