Exchange server 2013 proxy authentication

Hi All,

I wanted to know what is the recommended proxy authentication for exchange server 2013. 

I have observed the following from different proxy authentications:

1. Negotiate: By default, the proxy authentication is set to negotiate and uses anonymous as Log on network security(un-encrypted MAPI)

2. Basic: It always prompts for the user credentials and uses encrypted MAPI.

3. NTLM: Use negotiate at MAPI level by default

Please provide me some information on different authentication mechanisms and the one which is recommended to use. 

Thanks,

Srinivas.