In a nut, what makes Fantastical the best pick is its great design, superior natural language text entry, and its support for iCloud reminders. The only thing we dislike about Fantastical is its lack of an iPad version.
If you do not know about Fantastical, read the review1 and give it a go, especially if you’re tried of the Calendar App supplied by Apple. What I really miss there, is better integration with Reminders, but Fantastical takes great care of that. Plus, it’s got really great integration with URL-schemes which makes it pretty powerful in conjunction with e.g. LaunchCenterPro.