Hey there! Great question. In my opinion, the best privacy-focused phone operating system is definitely the Librem 5's PureOS - it's totally open-source and doesn't collect any of your personal data. Additionally, it comes pre-installed with a suite of security and privacy apps like Tor, PGP encryption, chat apps like Signal and Matrix (Riot), header-level ad-blocking with Mykrobe Privacy Firewall and permission management app Kitinerary to make using it straight forward for day-to-day life.
While Android also has some incredibly powerful open source custom ROMs developed by the community including AOSP Extended(AEX) Community OS based on stock android but adds usefull partys from other Group whitch risk your security sometimes So I recommend Librem 5. Overall Librem preserves privacy much more securely out-of-the box than all other third party alternatives to iOS or Google Play Store(Android). If you want more choice in flashing custom builds LineageOS-based builds can give greater extensibility toward retaining technology you may typically usein modern smartphones aside for example from microG’s recent full replacements to GApps APIs affected wih cloud-stored data sharing recommenations . But in my view, if we’re dealing with total beginners or experience tradeoffs for select LineageROMs then pureos stands ahead in Good selection Great Functions Necessary options most attractive Interface etc.security.