The Open Data movement triggered an unprecedented amount of data published in a wide range of domains. Governments and corporations around the world are encouraged to publish, share, use and integrate Open Data. There are many areas where one can see the added value of Open Data, from transparency and self-empowerment to improving effciency, effectiveness and decision making. This growing amount of data requires rich metadata in order to reach its full potential. This metadata enables dataset discovery, understanding, integration and maintenance. Data portals, which are considered to be datasets' access points, offer metadata represented in different and heterogenous models. In this paper, we first conduct a unique and comprehensive survey of seven meta- data models: CKAN, DKAN, Public Open Data, Socrata, VoID, DCAT and Schema.org. Next, we propose a Harmonized Dataset modeL (HDL) based on this survey. We describe use cases that show the benefits of providing rich metadata to enable dataset discovery, search and spam detection.