Biogas production is essentially based on organic materials and biological processes; hence it can contribute to the transition toward a biobased economy. Biogas is a biofuel that can contribute to a more renewable and local energy system. In comparison with other biofuels, biogas is more flexible and can be produced from many different types of feedstock, including biomass containing various shares of carbohydrates, lipids and, both from primary and secondary raw materials. However, a significantly expanded biogas production is dependent on good business conditions, in turn related to societal acceptance and support. There are many factors that can make a biogas solution more or less suitable for both producers and the broader society. Among the many influencing factors, the choice of feedstocks (biomass) for producing biogas and biofertilizer is of strategic importance. But, to assess the suitability is complicated, because it is linked to many different challenges such as cost, energy balance, environmental impacts, institutional conditions, available technologies, geographical conditions, alternative and competing interest, and so on. Suitability includes aspects related to feasibility for implementation, potential for renewable energy and nutrient recycling, and resource efficiency. In this article, a multi-criteria framework, which is proposed in a companion article (Part II), is used to assess the suitability of four types of feedstocks for producing biogas (considering Swedish conditions). The assessed feedstocks are ley crops, straw, farmed blue mussels, and source-sorted food waste. The results have synthesized and structured a lot of information, which facilitates considerably for those that want an overview and to be able to review several different areas simultaneously. Among the assessed feedstocks, biogas production from household food waste and ley is the most straightforward. For straw and farmed blue mussels, there are more obstacles to overcome including some significant barriers. For all feedstock there are challenges related to the institutional conditions. The assessment contributes to the knowledge about sustainable use of these feedstocks, and the limitations and opportunities for biogas development. It supports more informed decision making, both in industry and policy. Existing, or forthcoming, biogas and biofertilizer producers who are considering altering or expanding their production systems can benefit from a better understanding of different choices of feedstock that are or can be (potentially) at their disposal; thus, identify hotspots, weak points, and possible candidates for implementation in future. This research is performed within the Biogas Research Center (BRC), which is a transdisciplinary center of excellence with the overall goal of promoting resource-efficient biogas solutions in Sweden. The BRC is funded by the Energy Agency of Sweden, Linköping University, and more than 20 partners from academia, industry, municipalities and other several public and private organizations.