The Office365 authentication type and OrganizationServiceProxy Connection are now deprecated Link, which variant is the best to create a connection between dynamics 365 Online and c#. do you have any examples.
Personally I suggest to use Client Id & ClientSecret from an Azure App Registration. This method is also compatible with the .NET Core SDK package (still in Alpha https://www.nuget.org/packages/Microsoft.Powerplatform.Cds.Client/) as this package does not support username&password authentication.
You can find many tutorials online to use a Client Id & Client Secret, you can start from this one https://blog.magnetismsolutions.com/blog/paulnieuwelaar/2020/04/24/dynamics-365-xrmtooling-connect-with-azure-app-registration-in-c-