Most people go with either mineralized topsoil, or miracle-gro organic potting mix. The mineralized topsoil you have to prepare the dirt such as wash it then let it dry, then wash it and let it dry, this is done 3-5 times to enrich the soil with nutrients, and is better in the long run. With the Miracle-Gro Organic Choice Potting Mix all you really need to do is sift out any wood chips, and then it's ready to go.
Either way you decide to go, both work pretty well, I've done dirted tanks in the past with success but now I prefer to go with my substrate recipe which has work very well for me but runs around $50-$75 to create. It's a mixture of SeaChem Flourite plant substrate, FloraMax plant substrate, and some ADA Aqua Soil Amazonia...