Compost is used to improve lawn, garden and potting soil. The consistent structure and well composted physical state makes it ready to work into your soil. When blended into existing soil, compost will improve soil structure, increase organic content and provide a natural home for a rich population of beneficial biological characters.
It is virtually weed seed free and has a high organic content due to the care in managing the material.
Compost can be mixed directly into the garden soil or used as a mulching material that is mixed with the soil after the growing season.
Assuming that you’ve got average soil with average problems and you can only add one thing.
Compost would be the thing to choose.