Whichever system you can navigate through easily and freely, none of which is a smartphone. Smartphones are only temporary vessels on-the-go for calling, texting and photos/videos. Keep your computing as much as possible to a real, dedicated computer or laptop. Any mainstream Android phone in the past 3-4 years, if you do not root or unlock it, has been “secure” at this point, as long as you are not installing calculator apps that need your credit card info and camera access, and as far as your adversary is not the TSA airport agent with Israeli Cellebrite kit or you are not a state actor target for malware like Pegasus.
Funnily enough, Pixels have been horrifically insecure for a while now, besides their garbage QC issues. Google took months to fix these security issues for 6A, 7 series that were more easy to exploit than the security issues any other Android maker has had for the past few years.
Any decent Android phone post Android 9 version, provided you:
do not root or unlock it
you debloat it thoroughly
install apps carefully
put a firewall with nice DNS provider
restrict app permissions as much as possible
keep OTA security patches updated
is a secure phone to use. There is full disk encryption for years now, and iPhones are cheaper and easier to exploit than Androids since 5-6 years.
I have had a non-root smartphone guide for years now (lemmy.ml/post/128667), letting anyone have a private and secure Android device without any Safetynet tampering or bootloader unlocking complexity, which also allows to use Android Auto, bank apps and any of those Safetynet apps comfortably. This, to the best of my knowledge, is the Pareto frontier of usability, privacy and security on smartphones, provided you have an actual computer as well.
Someone made an Android app that allowed me to solve the issue of physical phone theft as well, effectively disallowing anyone (unless million dollar Cellebrite-like kits can exploit the stolen locked phone) to extract data out of your phone, in case someone took your phone on the street and ran away. This requires locked bootloader, which is the default state of any Android phone you purchase commercially, unless later unlocked or rooted.