Cash in the mail with Bisq is the most private. With CashApp and RoboSats you can share only your display name with the other party, but obviously you've already done KYC with CashApp and that info is available if the seller turns out to be an operative or CashApp gets subpoena'd.
Discussion
I don't really care about my identity being linked to the transaction. My current issue with KYC is that my ID is expired and so the stupid automatic ID scans that everyone uses doesn't go through. I'm probably not going to be able to renew my ID for another month or two, so I'm just looking for a way to purchase in the meantime.