#1 would be the phone app, pretty handy for calling people in real time. Without it I would have to carry a phone separately, It's become pretty hard finding a phone booth today.
#2 Would be SMS, these are quite handy for quick messages without interrupting something important.
#3 GPS maps, I have the most amazing sense of orientation, that basically guarantees I never end up where I’m supposed to, unless I have GPS.
You may take these functions for granted children. But back in the day, maps were something you had in the glove compartment of your car. Phones were something you had on the wall, with a long wire hanging out of them, to connect you to a wired network. Messages was something you send on paper, put a stamp on, and posted through the postal service.
Of course compared to back then, a smartphone has plenty more clever functions. Like the calculator, which we did either in the head or more complex calculations were done on paper.
But I guess these aren’t what you kids were looking for. But they are actually quite amazing, even if you take them for granted. ;)
Anybody without Maps on their list must take it so deeply for granted. Even for me, growing up printing MapQuest maps as a teenager, it's crazy how much before that was "whip out your massive dangerously distracting atlas and try to figure it out including where you are."
Yes camera is very cool too. Especially compared to buying film and then have to have them developed. Which was pretty expensive. The quality of pictures from a modern phone is even way way better.
I knew you meant apps besides "default" or the obvious, I just came to think how different it is today with smartphones, compared to before we had the first mobile phones. Which brings us back to the very early 80's. So I posted as mostly a joke, but also as a reminder that smartphones are pretty amazing IMO. ;)
PS. I'm from Denmark, and we were very early with first generation nationwide mobile network NMT already in 1982. And a phone for it could be had for as cheap as $7000. How time flies.