For whatever reason, finding quality potting soil is very difficult anymore. Most brands seem to harbor diseases, weed seeds, and bug eggs. And the nonstandard brands frequently appear to be nothing more than poor-quality topsoil dug up from the “manufacturer’s” backyard!
In light of this dilemma, we currently recommend Black Gold organic potting soil. Not just because it is organic (although that is certainly a plus), but primarily because several years of use have yielded satisfactory results in the form of healthy plants without unexpected weeds or small swarms of gnats flying around the seedlings.
Peat moss and forest humus provide a light, loose texture that works well for starting seedlings indoors. Compost and screened earthworm castings add just a little bit of all-natural nitrogen for a good start (of course, potted plants will still need to be fed periodically).
Shop around a bit before you buy—the prices do fluctuate and sometimes (but not always) a bigger size is a better bargain.