I would say for smartphones it is pixelphones with graphene os that are most private. If not try Calyx OS.
For Desktop is probably Cubes OS (that I personally never used and can not give any review on it therefore) or kind of any Linux distro that works for you, since they are all opensource and therefore have most transparency with privacy.
I personally use Fedora and would recommend it for sure.
But privacy is a complex thing. Since after the OS there comes applications too. If one tries to stick with opensource where possible this has already a big impact. But privacy means reasearch and work usually. Since one needs to change workflows where privacy is not at a good level already.
How about iOS?
Probably also one of the best options. And defnitly one of the easyest as well.
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