I've tried a significant number of Calendar apps. My favorites are Fantastical (for the reasons that members above have mentioned plus it has a URL scheme so I can use it in conjunction with LaunchCenterPro) and Week Calendar. Week Calendar has a nice week layout, intuitive zooming in on a day, and the best timezone support I've used. It allows you to follow the time zone of the phone (which is just convenient), but you can also set it to always ask you for the timezone when adding/editing. It's the only app I can remember that does this so easily. If you travel across timezones and need to enter appointments for other timezones, it helps a lot. (There are also icons, custom colors, etc., in Week Calendar, but I rarely use them.)