Google maps. I live in Japan and I don't read Japanese so it's extremelly helpful. Even when I was living in London 2 years ago, I was quite upset during the two months Google maps was not on iOS.
Even though I don't FB a lot, the App is quite handy to message someone that's not as tech savvy (or don't have a smartphone), especially in groups, and keep track of the convo history.
Ola cabs / Uber for India - I dont own a car so these apps were pretty much indispensable for me, but now they have been banned pretty much in many Indian cities.
- Good audiobook player
- Good ebook reader
- Good text editor
- Video player
- Torrents downloader
- Mindmapping app
- IRC app
- Wikipedia app
- Game of Go.
- Pomodoro(productivity)
If you're considering tablet apps - then also something for painting, like procreate.
Do you really use torrents on mobile phone?
Yes, and it is surprisingly convenient.
For what usage, if you have time to answer? Only things I need from torrents are TV series unavailable in my country, sometimes Linux distro.