iOS: Fantastical is one of the best calendars on the Mac. The iPhone version is just as solid, and features an intuitive interface, incredibly fast natural language input and lots of other useful features.
When you first launch Fantastical, you see the DayTicker, which is a weekly list you can slide left or right to view upcoming events. From that screen you can slide down slightly to search for events, or you can pull down a full monthly calendar to see everything going on. Sliding through the interface is actually kind of fun to use, and it's still fully functional as a calendar. As you would expect, Fantastical uses the calendars you have set up on your iPhone already (iCal, Google, Facebook), so if you're using something else, it will get populated with events automatically.
The best part of Fantastical is how quickly you can enter in new events. Simply tap the plus sign in the right corner, type something like, "Meeting at 2pm on Friday" and Fantastical does the rest. You don't have to jump through different boxes or menus just to add an event (although you can if you want to). It's a quick and simple way to add events without wasting time in menus.
Fantastical ($1.99) [iTunes App Store]