SET NOCOUNT ON;
USE tempdb;
GO
CREATE TABLE dbo.t (
  Id int IDENTITY,
  dt datetime NOT NULL
  );
GO
INSERT INTO dbo.t (dt)
  VALUES ('2018-01-01 15:30:40'), ('2018-01-01 21:30:40'),
    ('2018-01-02 20:30:40'), ('2018-01-02 22:30:40'),
    ('2018-01-03 05:30:40'), ('2018-01-03 08:30:40');
GO
PRINT 'SELEZIONE, per ogni data, della Riga con Orario Maggiore';
SELECT CONVERT(date, t.dt) [DateRef], MAX(t.dt)
  FROM dbo.t t
  GROUP BY CONVERT(date, t.dt);

PRINT 'Combinando la SELEZIONE precedente con la tabella base, possiamo estrarre le righe referenziate da ogni singolo orario';
WITH cte AS (
  SELECT CONVERT(date, t.dt) [Date], MAX(t.dt) AS [DateReferral]
    FROM dbo.t t
    GROUP BY CONVERT(date, t.dt)
  )
  SELECT t.*
    FROM dbo.t t
      JOIN cte c ON c.[DateReferral] = t.[dt]
    ORDER by c.[Date];

GO
DROP TABLE dbo.t;
--<---------
SELEZIONE, per ogni data, della Riga con Orario Maggiore
DateRef    
---------- -----------------------
2018-01-01 2018-01-01 21:30:40.000
2018-01-02 2018-01-02 22:30:40.000
2018-01-03 2018-01-03 08:30:40.000

Combinando la SELEZIONE precedente con la tabella base, possiamo estrarre le righe referenziate da ogni singolo orario
Id          dt
----------- -----------------------
2           2018-01-01 21:30:40.000
4           2018-01-02 22:30:40.000
6           2018-01-03 08:30:40.000