To start off with I think a browser and email client would be the most important.
Re:50 days of postmarketOS
by ReverseCold on Saturday July 15, 2017 @11:59AM
(#14777302)
Just get a browser and some form of networking to run and you've got a pretty usable OS for a lot of people.
This is me right now on my iPhone. Apart from an ssh app, a openvpn app, and my bank's app (which could probably be handled on the browser using KeyChain), I use the browser for everything. It's glorious.
My guess is that FB, Mail, Whatsapp, Twitter/insta/Snap, and news apps make up the top 5.