The lists of smartphones on the Postmarket OS and UBPorts websites can be used as a basis. Theoretically, you can use for experiments phones that come with alternative firmware directly from the factory. OnePlus, for example. You could buy a Pinephone and experiment with mobile Linux.

I prefer Android because we all do not have much choice. Once I preferred Symbian, it was an inconvenient system for advertising salesmen, then there was Windows mobile, I liked it too, but it died quickly. I like the pushbutton phones with proprietary firmware the most, but unfortunately these days it’s hard to live without mobile apps.