I personally think the answer should be 'none of the above'. The accuracy of sysdatetime() depends on the computer hardware and version of Windows on which the instance of SQL Server is running. Queries 2 and 4 may give an accurate value for the number of nanoseconds since when your server believes midnight to have been but not 'reliably' when midnight actually was unless you are confident that the date time of your server is accurate to 100 nanoseconds.