I agree - Reminders is my go-to application for most of my planning. I really like the integration between the Mac and iPhone - I do most of my creation of reminders on the Mac, and get reminded on the phone itself. I also share Reminders lists with my wife, like our grocery lists; makes it super easy to check whether or not she has already bought something.
This, besides the fitness tracker, will be the most important Watch app for me.