Fantastical meets all our criteria for a great calendar app, and we recommend it unreservedly for iPhone users of all ages, professions, and varying degrees of busy-ness. TimeTree is a shared calendar app meant to help you coordinate events in your calendar together with friends, colleagues, family, or whoever you need. I am using personal calendars on iCloud plus a work calendar on Exchange, and eM Client is almost the only program that can do that (yes, Outlook with the iCloud plugin works, but it is spectacularly ugly). The best calendar apps don't simply make it easy with set up and manage reminders, but will also integrate with office software platforms.