WITH cte_tbl AS (
   -- ваша исходная таблица с условиями
   SELECT 1 AS id, 'group1' AS grp, 1 AS b FROM rdb$database
   UNION ALL SELECT 2 AS id, 'group2' AS grp, 5 AS b  FROM rdb$database
   UNION ALL SELECT 3 AS id, 'group2' AS grp, 5 AS b  FROM rdb$database  -- Из группы group2 выведет эту строку
   UNION ALL SELECT 4 AS id, 'group2' AS grp, 3 AS b  FROM rdb$database
   UNION ALL SELECT 5 AS id, 'group2' AS grp, 2 AS b  FROM rdb$database
   UNION ALL SELECT 6 AS id, 'group3' AS grp, 2 AS b  FROM rdb$database
),
cte_max AS (
  SELECT t.grp, MAX(t.b) AS b_max
  FROM cte_tbl t
  GROUP BY t.grp
),
cte_res AS (
  SELECT t.id, t.grp, t.b, ROW_NUMBER() OVER (PARTITION BY t.grp ORDER BY t.id DESC) AS rn
  FROM cte_tbl t
    INNER JOIN cte_max m
      ON m.grp = t.grp
      AND m.b_max = t.b
)
SELECT *
FROM cte_res
WHERE rn = 1

------ Performance info ------
Prepare time = 15ms
Execute time =  4s 875ms 

------ Performance info ------
Prepare time = 16ms
Execute time =  406ms