I love Evernote - it’s a note taking app but I have it on my phone, iPad and laptop and it syncs across all of them so I always have what I’m looking for. I basically use it to keep track of my whole life!
My other recommended apps are What3Words which is more generally useful rather than life changing - it helps you identify your precise location, Dr Greger’s Daily Dozen which helps you track healthy habits, and Spotify.
I also used to have IFTTT which helps you automate various things on your phone which can be quite helpful.