Macworld:

The best iOS calendars in the field don’t just look great, they offer their own unique concepts, distinguishing themselves by how they handle and present our data. Whether it’s event input, intelligence, social integration, or notifications, calendar app makers have added a wealth of features and functionality to the stock iOS calendar, to the point where Apple has even begun to borrow popular features just to keep up.

My life isn’t busy enough to need a powerful calendar app but I have used and liked Fantastical in the past.