Any phone that can run GrapheneOS, which is arguably the most secure full-featured (as in: all the functionality you’d expect in a modern smartphone + compatible with popular mobile apps) mobile OS right now.

GrapheneOS is heavily focused on protection against attackers exploiting unknown (0 day) vulnerabilities. They employ techniques such as attack surface reduction (stripping out unnecessary code, disabling insecure components etc); using hardened system components (such as the kernel) that makes it much harder for hackers to exploit; and finally using sandboxing technologies (eg per-website browser sandbox, app sandboxing, media codec sandboxing etc).

A more interesting thing is the sandboxed Google Play Services support, which allows the option to use Google apps (such as the Play Store) in a fully sandboxed environment without granting them any special privileges.

You should check out the full feature set, it’s a LOT more impressive than what I hastily summarised above.

This focus on both privacy and security, with minimal negative impact to the user experience, IMO makes GrapheneOS probably the smartest choice for users concerned about mobile security and therefore, phones which run GrapheneOS (currently only Google Pixel phones) would be the smartest smartphone.