Windows 10 UAC prompt default to particular admin user
There seems to be no explicit setting/registry value for the default admin user that is being shown in the UAC dialog (the consent.exe process).
However, the order in which the admin accounts are displayed is quite simple: it's alphabetically.
The first alphabetically sorted account will be the default account the UAC dialog asks the password for. The accounts are sorted by username, not by display name.
So if e.g. you create an administrator account named Admin, it will be chosen by UAC over the Administrator account.
