Mainly I use the last, then the first two for details. *to set the pen pressure to control opacity, cs6 calls it transfer in the brush panel.
For sketching, since you use CS6 you'll have these brushes somewhere... I use one of the new dynamic brushes (I'm not quite sure they're called that). But anyways they're the ones were you can see the tip move on your screen. It's what most of the mixer brushes are made from. The brush image will be an icon instead of an image. I used one that looks like a triangular piece of chalk, and another that looks like a pencil with a sharp tip. I modified both a bit to my liking. My favorite is the second because it recognizes tilt very well and you can shade like you would shade with the side of the lead in real life.
I'm not sure what you mean by brush level? size? opacity? whatever you mean, just try each out and figure out what range works best for you. I constantly change the size of my brushes and my opacity settings vary and are in the 30-70% range usually.
95% of the time I'm using a hard round brush. My 'lineart' brush has a few settings to make the line crisper, but that's just because of the work I do needing crisp lines: it would be fine default for normal work. I use that brush for sketching, inking, and blocking in shading. My other hard round brush is set so that it's opacity and flow is linked to pen pressure: I also use that to sketch, rarely to ink, and to do soft shading. I have a couple more special brushes to make life easier when doing, say, water, but honestly a hard round brush with a few tweaks will do just about everything you need
Oh, I also like a square brush with the opacity linked to pen pressure.