1	ACCLTDEQNR	N	1150.55000
2	ACCLTDEQNR	B	1010.50000
3	HDFBANEQNR	N	0.00000
4	HDFBANEQNR	B	1097.25000
5	ICIBANEQNR	N	0.00000
6	ICIBANEQNR	B	834.20000
7	LARTOUEQNR	N	0.00000
8	LARTOUEQNR	B	2451.00000
9	TCSLTDEQNR	N	1001.70000
10	TCSLTDEQNR	B	1017.50000

1	ACCLTDEQNR	N	1150.55000
4	HDFBANEQNR	B	1097.25000
6	ICIBANEQNR	B	834.20000
8	LARTOUEQNR	B	2451.00000
9	TCSLTDEQNR	N	1001.70000

with 
  data as (
    select col1, col2, col3, col4,
           row_number () over (partition by col2 order by col3 desc) rn
    from tab
  )
select col1, col2, col3, col4
from data
where rn = 1
/

the second output ids based that if 'N' have then that value else the value of 'B'

col1 col2   col3 
 1     A       120
 1     B       140
 2     A         0
 2     B       165  
 3     A         0        
 3     B       156
 4     A       140
 4     B       125

col1 col2   col3 
 1     A       120
 2     B       165  
 3     B       156
 4     A       140

        select MWC_ESMM_SEM_SMST_SECURITY_ID,MWC_EXCH_ID,MWC_BUY_PRICE 
        from MKT_WATCH m
        where MWC_ESMM_SEM_SMST_SECURITY_ID in (select    CTQ_SECURITY_ID c FROM CC_TRANSACTION_QTY C
        where C.CTQ_CLIENT_ID = '330141')
        ORDER BY MWC_ESMM_SEM_SMST_SECURITY_ID DESC

 MWC_ESMM_SEM_SMST_SECURITY_ID	MWC_EX_ID   MWC_BUY_PRICE
1	TCSLTDEQNR	         N      	1001.70000
2	TCSLTDEQNR	         B       	1017.50000
3	LARTOUEQNR	         N      	0.00000
4	LARTOUEQNR	         B      	2451.00000
5	ICIBANEQNR	         N      	0.00000
6	ICIBANEQNR	         B      	834.20000
7	HDFBANEQNR	         B      	1097.25000
8	HDFBANEQNR	         N      	0.00000
9	ACCLTDEQNR	         B      	1010.50000
10	ACCLTDEQNR	         N      	1150.55000

with data as 
  (
  select 1 COL1,'ACCLTDEQNR' COL2,'N' COL3,1150.55000 COL4 FROM DUAL UNION ALL
  select 2,'ACCLTDEQNR','B',1010.50000 FROM DUAL UNION ALL
  select 3,'HDFBANEQNR','N',0.00000 FROM DUAL UNION ALL
  select 4,'HDFBANEQNR','B',1097.25000 FROM DUAL UNION ALL
  select 5,'ICIBANEQNR','N',0.00000 FROM DUAL UNION ALL
  select 6,'ICIBANEQNR','B',834.20000 FROM DUAL UNION ALL
  select 7,'LARTOUEQNR','N',0.00000 FROM DUAL UNION ALL
  select 8,'LARTOUEQNR','B',2451.00000 FROM DUAL UNION ALL
  select 9,'TCSLTDEQNR','N',1001.70000 FROM DUAL UNION ALL
  select 10,'TCSLTDEQNR','B',1017.50000 FROM DUAL
  )

select col1,col2,col3,col4 from 
  (select col1,col2,col3,col4,row_number()over(partition by col2 order by col3 desc)r
    from data where col4 <>0
  )
where r=1

with 
  data as (
    select MWC_ESMM_SEM_SMST_SECURITY_ID, MWC_EXCH_ID, MWC_BUY_PRICE,
           row_number () over (partition by MWC_ESMM_SEM_SMST_SECURITY_ID order by MWC_EXCH_ID desc) rn
    from MKT_WATCH 
        where MWC_ESMM_SEM_SMST_SECURITY_ID in (select CTQ_SECURITY_ID c FROM CC_TRANSACTION_QTY C
        where C.CTQ_CLIENT_ID = '330141')
        ORDER BY MWC_ESMM_SEM_SMST_SECURITY_ID,mwc_exch_id DESC
  )
select  MWC_ESMM_SEM_SMST_SECURITY_ID, MWC_EXCH_ID, MWC_BUY_PRICE
from data
where rn = 1

select MWC_ESMM_SEM_SMST_SECURITY_ID, MWC_EXCH_ID, MWC_BUY_PRICE,
           row_number () over (partition by MWC_ESMM_SEM_SMST_SECURITY_ID order by MWC_EXCH_ID desc) rn
    from MKT_WATCH 
        where MWC_ESMM_SEM_SMST_SECURITY_ID in (select CTQ_SECURITY_ID c FROM CC_TRANSACTION_QTY C
        where C.CTQ_CLIENT_ID = '330141')
        ORDER BY MWC_ESMM_SEM_SMST_SECURITY_ID,mwc_exch_id DESC

SELECT *
from MKT_WATCH 
where MWC_ESMM_SEM_SMST_SECURITY_ID in (select CTQ_SECURITY_ID c FROM CC_TRANSACTION_QTY C
                                        where C.CTQ_CLIENT_ID = '330141')

        select MWC_ESMM_SEM_SMST_SECURITY_ID,MWC_EXCH_ID,MWC_BUY_PRICE 
        from MKT_WATCH m
        where MWC_ESMM_SEM_SMST_SECURITY_ID in (select    CTQ_SECURITY_ID c FROM CC_TRANSACTION_QTY C
        where C.CTQ_CLIENT_ID = '330141')
        ORDER BY MWC_ESMM_SEM_SMST_SECURITY_ID DESC

 MWC_ESMM_SEM_SMST_SECURITY_ID	MWC_EX_ID   MWC_BUY_PRICE
1	TCSLTDEQNR	         N      	1001.70000
2	TCSLTDEQNR	         B       	1017.50000
3	LARTOUEQNR	         N      	0.00000
4	LARTOUEQNR	         B      	2451.00000
5	ICIBANEQNR	         N      	0.00000
6	ICIBANEQNR	         B      	834.20000
7	HDFBANEQNR	         B      	1097.25000
8	HDFBANEQNR	         N      	0.00000
9	ACCLTDEQNR	         B      	1010.50000
10	ACCLTDEQNR	         N      	1150.55000

create table simple_test (MWC_ESMM_SEM_SMST_SECURITY_ID varchar2(30),MWC_EXCH_ID varchar2(1),MWC_BUY_PRICE number);

insert into simple_test values ('TCSLTDEQNR','N',1001.70000);
insert into simple_test values ('TCSLTDEQNR','B',1017.50000);
insert into simple_test values ('LARTOUEQNR','N',0.00000);
insert into simple_test values ('LARTOUEQNR','B',2451.00000);
insert into simple_test values ('ICIBANEQNR','N',0.00000);
insert into simple_test values ('ICIBANEQNR','B',834.20000);
insert into simple_test values ('HDFBANEQNR','B',1097.25000);
insert into simple_test values ('HDFBANEQNR','N',0.00000);
insert into simple_test values ('ACCLTDEQNR','B',1010.50000);
insert into simple_test values ('ACCLTDEQNR','N',1150.55000);

with 
  data as (
    select MWC_ESMM_SEM_SMST_SECURITY_ID
          ,MWC_EXCH_ID
          ,MWC_BUY_PRICE,
           row_number () over (partition by MWC_ESMM_SEM_SMST_SECURITY_ID order by MWC_EXCH_ID desc) rn
    from simple_test
    ORDER BY MWC_ESMM_SEM_SMST_SECURITY_ID,mwc_exch_id DESC
  )
select  MWC_ESMM_SEM_SMST_SECURITY_ID, MWC_EXCH_ID, MWC_BUY_PRICE
from data
where rn = 1;

MWC_ESMM_SEM_SMST_SECURITY_ID  MWC_EXCH_ID MWC_BUY_PRICE          
------------------------------ ----------- ---------------------- 
ACCLTDEQNR                     N           1150.55                
HDFBANEQNR                     N           0                      
ICIBANEQNR                     N           0                      
LARTOUEQNR                     N           0                      
TCSLTDEQNR                     N           1001.7

1	ACCLTDEQNR	N	1150.55000
4	HDFBANEQNR	B	1097.25000
6	ICIBANEQNR	B	834.20000
8	LARTOUEQNR	B	2451.00000
9	TCSLTDEQNR	N	1001.70000

I thing you have not understood the requirment of writing this query 

with 
  data as (
    select MWC_ESMM_SEM_SMST_SECURITY_ID
          ,MWC_EXCH_ID
          ,MWC_BUY_PRICE,
           row_number () over (partition by MWC_ESMM_SEM_SMST_SECURITY_ID 
                               order by case when mwc_exch_id = 'N' and mwc_buy_price > 0 then 2 
                                             when mwc_exch_id = 'B' then 1 
                                             else 0 end desc) rn
    from simple_test
    ORDER BY MWC_ESMM_SEM_SMST_SECURITY_ID,mwc_exch_id DESC
  )
select  MWC_ESMM_SEM_SMST_SECURITY_ID, MWC_EXCH_ID, MWC_BUY_PRICE
from data
where rn = 1;

MWC_ESMM_SEM_SMST_SECURITY_ID  MWC_EXCH_ID MWC_BUY_PRICE          
------------------------------ ----------- ---------------------- 
ACCLTDEQNR                     N           1150.55                
HDFBANEQNR                     B           1097.25                
ICIBANEQNR                     B           834.2                  
LARTOUEQNR                     B           2451                   
TCSLTDEQNR                     N           1001.7        

  1  with
  2    data as (
  3      select MWC_ESMM_SEM_SMST_SECURITY_ID
  4            ,MWC_EXCH_ID
  5            ,MWC_BUY_PRICE,
  6             row_number () over (partition by MWC_ESMM_SEM_SMST_SECURITY_ID order by MWC_EXCH_ID desc) rn
  7      from simple_test
  8      where mwc_buy_price > 0
  9      ORDER BY MWC_ESMM_SEM_SMST_SECURITY_ID,mwc_exch_id DESC
 10    )
 11  select  MWC_ESMM_SEM_SMST_SECURITY_ID, MWC_EXCH_ID, MWC_BUY_PRICE
 12  from data
 13* where rn = 1
SQL> /

MWC_ESMM_SEM_SMST_SECURITY_ID  M MWC_BUY_PRICE
------------------------------ - -------------
ACCLTDEQNR                     N       1150.55
HDFBANEQNR                     B       1097.25
ICIBANEQNR                     B         834.2
LARTOUEQNR                     B          2451
TCSLTDEQNR                     N        1001.7