Watercolors
I am currently using Windsor & Newton™ Cotman Water Colours for my watercolor paintings. I use a wide variety of brushes, though for most of theses paintings I used a round white synthetic brush.
Most of these are created in layers of paint, combined with Windsor & Newton™ Water Colour Colorless Art Masking Fluid. This works a bit like rubber cement. It is applied with a brush you don't care about anymore (it's hard to wash out!), a thin brush handle tip or even a palette knife for very thin lines. It does pull up the color if you apply it over paint, so I have found you have to judge your color layers accordingly.
I have been very excited with the results of these and have had great feedback so far.