iCalendar

Seems too complex for anyone to properly implement based on my frustrating experiences with bad parsers and malformed event data seen in the wild. Using a trivially parsable format like XML with a well-defined schema will address this problem. 

hCalendar

Is easier to work with, but often suffers because it's used for both event content and website UIs, so changes to UIs often damage the event content data. Offering the data separately from the UI will address this problem.