What is the hierarchy of aviation regulatory bodies?
As pointed out by vasin1987 in a comment to the question, each country's regulatory body has control of that country's airspace: These agencies exist in a "mesh" of coequals as opposed to a hierarchy.

The EASA is a EU agency and applies to all of EU, very similar to how the FAA is a US agency that applies to all of the US.

ICAO, in turn, deals primarily with situations where air traffic crosses regulatory borders. In its own words, borrowing from the page About ICAO on their web site,


  The International Civil Aviation Organization (ICAO) is a UN specialized agency [which works] to reach consensus on international civil aviation Standards and Recommended Practices (SARPs) and policies[.]


Like pretty much the rest of the UN, the ICAO is more a forum for cooperation between regulatory bodies than a regulatory body in its own right.