I think the most under-appreciated additions to iOS9 are definitely the enhancements to Reminders (contextual and location).
Maybe that's just me though. At this point, dictating reminders to Siri is so easy that I basically use it as a sort of hybrid calendar and to-do list. This has been the case for awhile. But the new tools are wonderful, location-based reminders especially.
The contextual stuff is more impressive on the surface, I think: If you are looking at a tweet or a web page or something else, you can invoke Siri and say something like "Remind me about this in three hours." Siri will then link directly to whatever you were looking at in the reminder itself.
The location stuff is much simpler and location gating is nothing new, but something as simple as "remind me to ______ when I get home" is just mind-bendingly useful to a forgetful slob like me.
Maybe I am the only crazy person abusing the Reminders app like this. But seriously, if you're like me, swamped with things to remember but you find keeping a calendar way too tedious, you should give voice-dictated reminders a shot. For me, it was life-changing.