Miso paste

For soups, I would go with a light (yellow or white) smooth miso. 

I second mymonstersprotectme's advice of buying one that comes in a tub, much easier to scoop miso out of, and easier to close back up. 

Hikari miso is widely available and is good quality IMO. 

My favourite miso ever was the miko shiro miso but I can't find it anymore :[ so I'm using a hikari shiro miso now.

The secret to make good miso soup is not only the miso, but the dashi stock.
My lazy way of making dashi is: 

I keep a jar in the fridge in which I put some 

- water, 

- kombu kelp, 

- wakame seaweed, and 

- a couple of dehydrated shiitake mushrooms. 

After the mushrooms have rehydrated and the water has thickened, it's ready, and a jar lasts me a week. 

This serves as a base for miso soup, but also for other easy dishes such as oyakodon.
I mix some cold dashi from the fridge with boiling water, which helps lower temperature (boiling water kills miso), add the miso paste, some soy sauce, and sprinkle some dried wakame (keeps forever in a ziplock) and frozen chopped scallions (I buy them fresh, chop a bunch, freeze, and have enough to garnish food for a long time).

You can also customize miso with tofu, mushrooms, carrots, daikon raddish, etc. if you make the authentic version of miso soup with hot dashi over the stove.