I see you are a newer user here - welcome to Android Central!
I have a few favourite apps. First off is Fitbit, especially since I have Fitbit Premium and can access all of my stats and workouts within the app. It's probably one of the most useful apps I have.
Next up is Zedge. It's been an app I've used for years. It's gotten me awesome wallpapers, notification tones, ringtones, and it's free! Great selection there.
I'm a moderator for a TV show page on Facebook, and so I use a few photo editing apps. I use Polish and PhotoRoom. Both are really excellent.
Can't wait to see other users' responses to this thread! Great topic!