If the internet can be believed, Graphene is the absolute holy grail. That's why people buy Pixel phones specifically so they can flash Graphene.

Have a look at some of the threads on !privacyguides@lemmy.one for comparisons, my read on it is that GrapheneOS is trying to be as secure as possible which brings a lot of privacy with it, and CalyxOS is a bit more permissive because they're just trying to degoogle. What this means is that only Pixels meet GrapheneOS's standards whereas CalyxOS will run on other devices, and CalyxOS uses MicroG to spoof Google Play Services while GrapheneOS comes without any but allows you to install a sandboxed version of GPS if you need it.

You can have a read though GrapheneOS's website to get the pretty in-depth details on what it does, why it does it, and how to do it yourself. I've been using it for a couple of years now and I'd recommend it, happy to answer any questions if you need!