The iOS Reminders app easily sees the most day-to-day use for me in the daily management/productivity realm. Many times a week I'll just say "Hey Siri, set a reminder for 7pm tomorrow to do X" and then I can just forget about it. My memory's not the greatest, so I find that offloading even the smallest tasks to the Reminders app has helped me immensely. :)

For longer-term events, I used to be a heavy Google Calendar user while in college, but now I typically just throw events onto the Reminders app as well! I like to set them for a certain amount of time before the event in case I need to remind myself to accomplish a few sub-tasks in the days/hours beforehand. For example, if I'm flying out to a different city a month from now, I'd schedule a reminder about that flight for a few days before the day of the flight, and when it goes off I know to create some reminders like "do laundry" for the next day and "check in for flight" for the night before.

Of course, sticking with a calendar is probably the better approach for this entire thing. :P I still pop things on my calendar, but I just like the sense of checking items off once they're done.