On most models the standard brush I'd use would be the GW basecoat brush, the detail brush and the fine detail brush. I also use a Series 7 00 for eyes. If I were talking in the gauge terms of Series 7 I guess I'd also use a 2 and a 4 (I have a 2, but it's more expensive than I need for everyday purposes so I keep it for special occasions.)
At the end of the day the stated gauge of the brush isn't important, but the snap and fineness of the point as what create the main factors you should be looking at. Often, if you can achieve the same point with a "larger" brush it's better because you can load with more paint; allowing longer and more strokes before you have to top up and reform the point.