Organic matter is what you are looking for to add to your soil to improve it, any well composted organic matter will do the job. That said, some are better / easier to use than others.
Our favourite is a good quality screened organic compost or green compost, the best being PAS100 certified organic compost, which is produced in a monitored proccess, ensuring there are no weed seeds or pathogens within the compost. We also recomend ensuring it has been well screened (sieved) to remove larger particles and make it much easier to dig in to your existing soil than some other types of compost (i.e manure).