Android. As bad as it is, if I had to use iOS or Linux phones it would be even worse, at least with the current state of Linux phones.
But actually, maybe if Android didn't exist, the FOSS community would focus more on Linux phones and they would be an actually good option. Maybe Android shouldn't exist?
For me it’s iOS, funnily enough. I use Windows for all of our video game machines and Linux for everything else, but I don’t use any Google products or services. After messing around on my computers all the time, I don’t want to even have to THINK about doing things to my phone to make it go. My current phone is six years old and the only reason I’m upgrading this year is to get a 120hz screen, USB-C, and for better low light pictures of cats. And a terabyte would be nice.
Google is a bad company, and Apple isn't any better. Probably the best option for you would be GrapheneOS on one of the latest pixels, they have intuitive software, 120hz screens, have had USB-C for years, a good camera, lots of storage, and most importantly GrapheneOS doesn't use Google or Apple, it's FOSS.
GrapheneOS is awesome, but like I said, no google products and I don’t want to fuck with my phone at all. Apple isn’t perfect, but it’s leagues better than stock Google with app permissions and overall privacy. My six year old phone is still fully supported for at least another year, and I enjoy the OS for the very few things I do on my phone. This is definitely the best option for me.