Dexron III is the current spec for older GM transmissions. It covers all the older Dexron specs. Dexron VI is a lower viscosity fluid, designed for modern transmissions.
Any name brand will do, so long as it meets the Dexron III spec. I have a preference for Mobil and Valvoline products, if I'm not using synthetics (I use Red Line Oil products in newer vehicles, mostly because that's what I have in my shop for a range of older to newer BMWs I work on), but that's just a personal preference.
