Items in Win Rate Order
About this guide
Data from http://aram.lolalytics.com/champion/Janna/ on December 1 2016 (patch 6.23)
Use the ideal build as a guide. Adapt your build based on the circumstances. For example, if the other team is all AP, Banner of Command is a much better item than usual.
An interesting note is that Janna is sometimes built full AP (Q max) for instant waveclear and built ADC. Neither of these builds are as good as the standard Janna build, especially not AD, but they are possibilities if your team completely lacks either of these types of damage.