The very best soil I know of is any soil my tobacco plants have their roots stuck in
Seriously this is actually a very important question.. i have found my plants to be very reactive to how wet the soil is. the make up of your soil makes a big difference. So far I have a soil that is far more sandy than I would want for most other things I have grown. I want a lot of compost or fiberous material to hold some moisture, but not hold actual water. hope that makes since. As others put it. Damp, not wet. and the longer you can get it to stay damp helps in how hard you have to work. some of the stuff you might think of for keeping soil damp like peat moss. have additional negative effects like the wrong Ph. so just careful that one answer is not causing the next problem. Not at all really difficult so far, but still one thing to pay attention to.
My soil is one part compost mixed with one part peat moss. this mixture is than mixed one to one with sand. so I now have a 50% sand, 25% compost 25% peat moss mix. this is then mixed 2 parts mix with one part dirt form my yard (for silt and clay) this results in a shake test that indicated the desired amounts of sand, loam and clay mixture I am looking for. A shake test means to take a handfull of yoru soli. put it in a clear jar of water, shake it up and then let everything settle to the bottom. you can then see the seperate layers of what your soil is made up of. bottom layer is sand, middle layer is silt, and top layer is clay. you want a mixture that is about equal amounts of sand a silt with about half as much clay as you see in either the sand or silt. so roughishly 40% sand 40% Silt/loam and 20% clay. In my case I am silt poor and wil have to allow repeted mixing of compost to eventually build it up. that will have to be added every year until the silt content is where I need it to be.