What are the best cross-platform calendar apps?

All views have an option of adding a new event by tapping the plus sign at the lower right corner of the screen. This opens up a form and allows the user to add a location, start/end times, guests, description, color-code as well as choosing if it's an all day event, how often it repeats, how many and what kind of reminders are needed and setting the users availability and privacy settings.