uh,
just one app?
not even close to conceivable.
in no particular order,
Evennote
the dialer
most important though is Nova Launcher, which gives me multiple pages, and Folders to keep similar apps in...
GPS Equalizer,
multiple GPS related tools
multiple WiFi tools
AquaMail for email
Textra for SMS
ASUS File Manager for sending files from the phone to my laptop's Shared folder.
Firefox for the browser
and Google's many apps which I use multiple times a day.
( yeah, I don't give two hoots in hell if they track me, nothing here for them to use )
Park My Car ( gps app ), that damn car constantly moves itself in those huge metro parking lots and refuses to come to me when I whistle for it, at least my dog knows to come when I call.
GasBuddy, saves me a lot of moola every time I fill a gas tank
bank apps
if I had to pick just one app, it would be
MacroDroid
a very powerful tool that can do anything you can conceive of, I use it to tell the Car Warranty robots to go where hell freezes.
plus a lot of other stuff, I have 13 macros set up.
ColorNote, Fuelio ( keep records of gas used, mntc etc. )
Tree identifier, plants, etc
Clinometer, measure road grade inclines.
more mundate stuff...