I use Lineage OS (https://lineageos.org) with the cameras taped. It's not perfectly secure, but it's the best tradeoff I can make at the moment. Here's a few better options I want to switch to in the future:

Graphene OS, a security-hardened Android ROM: https://grapheneos.org

Librem 5, probably the ideal solution: https://puri.sm/products/librem-5/ Just look at the bullet-point features:

  + Does not use Android or iOS. The Librem 5 comes with the mobile version of our FSF-endorsed operating system PureOS by default, and is expected to be able to run most GNU+Linux distributions.  
  + CPU separate from baseband, isolating the blackbox that the modem may represent and allowing us to seek hardware certification of the main board by the Free Software Foundation.  
  + Hardware Kill Switches for camera, microphone, WiFi/Bluetooth, and baseband.  
  + End-to-end encrypted decentralized communications via Matrix over the Internet.  
  + We also intend the Librem 5 to integrate with the Librem Key security token in the future.

Pinephone, not as good as the Librem 5, but much cheaper: https://www.pine64.org/pinephone/