Chronos
Cal
Super Calendar
Week View 8
Iconical Plus
True Calendar
Office365 Calendar
VsysCalendar365
Calendar Calleage
Simple Calendar
They all have their own pro's and cons...like Iconical Plus is the only calendar app (exept the in-house app off course) that syncs outlook tasks, and the only one that gives seperate live-tiles for every "view" AND for tasks seperately.
Super Calendar then has many good features and a very unique UI setup (even has two UI's you can choose from, one very snappy and the other a bit slower but very elaborate). I bought this app because of the awesome support you will be getting. The dev of SC is awesome. The price is pretty steep, but you will be getting very very good support for the extra buck.
Simple Calendar actually is pretty amazing for a free app. B sure to check it out aswel.
Cal is simply beautiful to the eye, VERY snappy, and fits the WP style perfectly, but has very few features compared to the others mentioned. Very minimalistic, but functional none the less.
Chronos has a UI that doesn't fit the WP look at all (even find it ugly personally), but has quite some features and since last update has about 100 visual styles/colors for adjusting the UI a bit to your taste.
True Calendar imo has the most beautiful look, but again that is personal pref.
Office365 Calendar blends best with outlook and office, but beware cause although it is marketed as "free", it isn't. If you want to sync with outlook, you'll have to do an in-app purchase, which is quite ironic.
VsysCalendar is actually just a "variant" of Office365 calendar (they are built by the same dev).
Calendar Calleage is perfect for business men that want to be able to dial in to their meetings.
These have both live-tile and lock screen support : Super Calendar, Simple Calendar, Cal, Calendar Calleage, Iconical Plus
These only have live-tile support : Chronos, Week View 8, Office365 Calendar, VsysCalendar365
True Calendar has both, but you can only have back-tile when lock screen is disabled !
Etc...etc...
In the end, the only thing that truly seperates these apps is personal preference.
Try them all and decide which one is for you ;-)