I generally prefer "hana katsuobushi" because they are larger flakes of katsuo (bonito) and are easier to strain than the finer katsuobushi that's used for a topping. I don't have a preference as to brand.
As far as konbu is concerned, I specifically use "dashi konbu" because it's cheaper than other konbus that are meant to be eaten and are thinner, better quality and more even pieces. Again, I do not have a preference as to a brand.
I've found that it's important NOT to have the katsuobushi and konbu come to a boil because it will cause the broth to become bitter. The technique and quantity of ingredient used is more important than the quality of konbu and katsuobushi in my experience.
As far as powders go, I prefer to use the packets that do not contain msg (they are more like tea bags actually). They can be made simply with konbu, or katsuobushi or even niboshi (dried sardines). The niboshi packets are definitely stronger flavored. They're fine for me when I make miso soup.