What are the default fixed server role members for SQL Server 2012
On a clean install, just do:

SELECT role_name = pr.name, member_name = m.name
FROM sys.server_role_members AS r
INNER JOIN sys.server_principals AS m
ON r.member_principal_id = m.principal_id
INNER JOIN sys.server_principals AS pr
ON r.role_principal_id = pr.principal_id;


Here's what I get:

sysadmin    sa
sysadmin    CANUCK\aaronbertrand -- Administrator account I added during setup
sysadmin    NT SERVICE\SQLWriter
sysadmin    NT SERVICE\Winmgmt
sysadmin    NT Service\MSSQL$InstanceName
sysadmin    NT SERVICE\SQLAgent$InstanceName