Oracle 18c

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

WITH t(site_id, volume, rpt_date, rpt_hour) AS
(
 SELECT 1, 10, date'2021-01-01', 1 FROM dual UNION ALL
 SELECT 1,  7, date'2021-01-01', 2 FROM dual UNION ALL
 SELECT 1, 13, date'2021-01-01', 3 FROM dual UNION ALL
 SELECT 1, 11, date'2021-01-16', 1 FROM dual UNION ALL
 SELECT 1,  3, date'2021-01-16', 2 FROM dual UNION ALL
 SELECT 1,  5, date'2021-01-16', 3 FROM dual UNION ALL
 SELECT 2,  9, date'2021-01-01', 1 FROM dual UNION ALL
 SELECT 2, 24, date'2021-01-01', 3 FROM dual UNION ALL
 SELECT 2, 16, date'2021-01-01', 3 FROM dual UNION ALL
 SELECT 2, 18, date'2021-01-16', 1 FROM dual UNION ALL
 SELECT 2,  7, date'2021-01-16', 2 FROM dual UNION ALL
 SELECT 2,  1, date'2021-01-16', 3 FROM dual
)
SELECT DISTINCT 
       site_id, 
       MAX(volume) KEEP (DENSE_RANK FIRST ORDER BY volume DESC) 
                   OVER (PARTITION BY site_id, rpt_date) AS volume,
       rpt_date,          
       MAX(rpt_hour) KEEP (DENSE_RANK FIRST ORDER BY volume DESC) 
                     OVER (PARTITION BY site_id, rpt_date) AS rpt_hour
  FROM t
 GROUP BY site_id, rpt_date, volume, rpt_hour 
 ORDER BY site_id, rpt_date

SITE_ID	VOLUME	RPT_DATE	RPT_HOUR
1	13	01-JAN-21	3
1	11	16-JAN-21	1
2	24	01-JAN-21	3
2	18	16-JAN-21	1