What is the best type of account to use for SQL services?
I generally start with AD accounts because someday, someone's going to want to join this server into an Availability Group. Then, they're going to wonder why they have problems with Kerberos authentication. Books Online states:


  All server instances that host an availability replica for the availability group must use the same SQL Server service account.


So I just head that off at the pass by using an AD account for any SQL Server, and then if I end up needing to add it to an AG later, all I have to do is set up the other replica with the same service account.