A tip from Mark Bittman:
To make a creamy vegetable soup: use three parts liquid (broth, stock, water), two parts vegetables (cooked and pureed), and one part dairy (cream, milk, yogurt, etc.)
Root vegetables work well for this, but other vegetables can be used. I heartily recommend celeriac, as it adds a creaminess that is difficult to match. Cooking the vegetables in the stock (or other liquid) will help to ensure that you don't lose any flavor.