Ask HN: What do you think is the Best Open Source Calendar?

A lot of good men have died on the hill of solving the enterprise shared calendar problem... Can we all share some info about this to focus efforts better?

Who knows what the best open source calendar projects are that syncs well with other cal systems, are mobile friendly and/or mobile first? Responsive design would be a plus but now I'm just being greedy.

I'm personally a fan of Fantastical for my personal use but it's far from perfect and not something I can write into our enterprise application.

Thoughts?