I will add more brushes as I come across them but here is a line of brushes I have fallen in love with :
I bought a few of these brushes and had a chance to try the fan brush. It has become my favorite. Their fan brush is almost as stiff as a hog bristle brush and retains its shape well. I, mostly, paint sceneries with a lot of grass and this brush covered a lot of area in a very short time.
One thing to keep in mind when using their fan brush (like all synthetic fan brushes) is to make sure the paint is not very thick. The brush bristles tend to clump up (more than natural bristle brushes) when there is not enough water in the paint.
The next line of brushes I am going to try (specifically fan and round brushes) are these: http://www.dickblick.com/products/princeton-series-6300-best-synthetic-bristle-brushes/#items. The Princeton 6300 series of brushes is supposed to be as stiff as the natural hog bristle brushes.