Sunrise

Sunrise is the only calendar app you'll need for Google Calendar, iCloud, Exchange. That's all well and good, but the interface is gorgeous and functions like you want it to. Quick Add Event just by typing "Lunch tomorrow at 9pm" (long-press on the +) (for Google Calendars only). I particularly like the faces included, such a simple yet appreciated feature. 

Moving to a new calendar is a hassle, but Sunrise makes the transition effortless.