Oracle 18c

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
CREATE TABLE mytable(
   ROOM_NO    INTEGER  NOT NULL 
  ,BRANCH_ID  VARCHAR(4) NOT NULL
  ,COST_MAINT INTEGER  NOT NULL
);

begin
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (1,'B001',23500);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (2,'B001',3750);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (2,'B001',4000);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (3,'B001',23000);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (5,'B001',5300);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (9,'B001',25000);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (43,'B002',2500);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (44,'B002',5300);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (48,'B002',3750);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (49,'B002',3000);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (49,'B002',3750);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (53,'B003',4000);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (56,'B003',4000);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (68,'B003',4000);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (75,'B003',25000);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (85,'B004',5800);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (86,'B004',3000);
INSERT INTO mytable(ROOM_NO,BRANCH_ID,COST_MAINT) VALUES (120,'B005',25000);
end;
/
1 rows affected

select 
    max(room_no) keep(dense_rank first order by cost_maint desc) as room_no,
    branch_id,
    max(cost_maint) as cost_maint
from mytable
group by branch_id


ROOM_NO
BRANCH_ID
COST_MAINT


9
B001
25000

44
B002
5300

75
B003
25000

85
B004
5800

120
B005
25000