Trying to strike a balance between privacy, usability, stability, and price I'm finding it hard to find a decent option.
If you're worried about advertisers - you have two choices. On any phone you can root, replace the HOSTS file using https://adaway.org/
On phones you cannot root, use https://blokada.org/index.html to create a tracker blocking VPN.
Run Firefox as your main browser. Use sandboxed apps for Facebook, Twitter, etc.
In terms of smartphones the Blackberry KEYone claims to be the most secure Android Smartphone.
(disclaimer: I have not used this smartphone and I am not a security specialist.)