GrapheneOS seems to be consistently the hardest to exploit AFU based on various Cellubrite leaks. iPhones have better protection than all other Androids except Pixels.
I might be wrong, but I get the impression that the GrapheneOS folks generally recommend GrapheneOS > iOS > Pixel >> everything else.
It might have to do with e.g. Apple having rolled out MIE at a broader scale than Google rolling out MTE on PixelOS, where AFAIK it is still largely opt-in (not 100% sure, I always wipe a Pixel immediately).
There are more recent leaks. The last release of GrapheneOS they've been able to exploit on locked device is still from 2022 as of a couple months ago. They take longer to break into iPhones than stock Pixels but that may largely be due to Google giving much earlier access for public testing. They have a far shorter window to prepare for a new iOS release before it's in production as a regular update for users.