The easiest security to implement is a lack of security, though not recommended at all:
<message clientCredentialType="None" />
<transport clientCredentialType="None" />
The next, which actually provides some security, would probably be Windows
(see a tutorial on enabling this here) or Password
for transport level security. See here for an MSDN overview of available credential options.
But to be honest there is quite a broad area for you to cover other than just specifying easiest. You can combine Transport
and Message
security, go with either or, use different credential types and so on - really, the easiest depends on the requirements, which, given point 5 of your question, would seem to reinforce my recommendation of Windows
or Password
authentication for this scenario.