These are the ones I use most when I paint. ^-^ Thought someone might find one useful!
The first is really good for blending things like skin, clouds, and shadows to make them softer. If you set it to 90/100% opacity it's pretty good for laying down colours too.
The second is basically a tweaked version of the pen tool. It doesn't blend but it's really good for laying down colour at every opacity.
The third (I think the flat shaped brush is a default shape that comes with SAI?) is nice for a rougher look. It blends really well and I really like the effect you get when you use it to erase around the edges with it (press the "-" key on your keyboard to turn any of the drawing tools into an eraser