We are down to just two operating systems for mobile phones (tablets too) Android and iOS.
Android is controlled by Google, no OEM phone maker that wants to do business globally will defy Google and try and fork it. Android and a lot of the most popular apps tells Google everything it can about you and your every move. In otherwords it leaks your privacy out like a sieve.
iOS is proprietary from Apple. It’s a walled garden. You do it Apple’s way or STFU. It is probably more private than Android. And it works.
That’s it. No other choices.
Long term the only other chances for a mobile OS come from Linux and here are the ones I’m aware of.
Sailfish – spun off from Nokia’s flirtation with Linux, this one does not seem to be gaining traction. And it seems like the US is always being left out of release plans. No OEM has adopted it. You can download it and try and install it on a couple of old model compatible phones.
KDE Plasma Mobile – it’s hard to tell how far along this Linux based OS is. The screen shots are nice. (See notes for Librem 5 below.)
UBports Ubuntu Touch – This community effort seems to be making big strides. When Ubuntu gave up on Ubuntu Mobile they turned it over to a volunteer community UBports who have been working away ever since. It comes with a couple hundred apps and web apps, plus anything that the browser can handle. If I were a phone OEM, I’d have my eye on this. You can download this now and install it on several old model phones. Some have an installation wizard. (See notes for Librem 5 below.)
Puri.sm Librem 5 – this is actually a real phone hardware not just an OS. The OS is Linux adapted to mobile. The last I heard, the plan is that the Librem 5 will come with Purism’s mobile OS installed by default, but it will be fully compatible with UBports Touch and Plasma Mobile. No word on how easy it will be to install any one of these. Launch has been delayed from January 2019 to April 2019. I’m hoping this is the point of the spear and proves very successful so that other OEM’s become interested in Linux phones.
Linux needs to get into mobile where all the growth is. It can’t just stay on the desktop. There may be others but these seem to be the furthest along.