GrapheneOS is the most privacy-focused of them all, but since it only supports Google devices, using it would mean directing money to a massive privacy violator and using hardware that they control. So it's not necessarily the best choice for everyone, even if they can afford one.

Depending on your needs (have you thought about your threat model?) LineageOS without Google Play services installed might be good enough. Lots of devices are officially supported, and unofficial ports can often be found for other devices on XDA Forums. I do this, building from source code myself instead of trusting pre-made builds.

If you go that route, keep in mind that non-Pixel devices generally don't allow installing your own keys and re-locking the bootloader, so an adversary that gets their hands on your phone would most likely be able to install malware on it. This is not a big concern for me, because I don't leave my phone anywhere that could happen, and I would consider even a Pixel to be compromised if an invasive agency like the TSA got physical access to it.