This blog post on The Verge reviews some of them and finally it recommends Sunrise. Here is the video that explains why you should choose Sunrise:
A calendar app is a great way to organise your activities and you don’t realise how useful it can be unless you start using one. For starters you can use the Google calendar that pretty much space to any environment and operating system. Most of the calendar apps for your iPhone must be able to sync with the Google calendar.
My personal favourite is, of course, Google Calendar. This is because I use my calendar less on the phone and more on my computer and if you use the default Google calendar application on a smart phone, it seems a bit cramped. You can try iCal from the makers of Any.do, that makes the Google calendar entries a bit sexier but in the end, nothing matches a computer screen when you want to use a calendar. Or at the most, you can use a tablet like an iPad. For a phone, a task list is much better.