create table t1 (A  number, B varchar2(10));

insert into t1 values (10 ,'Pa');
insert into t1 values (10 ,'Pb');
insert into t1 values (10 ,'Pc');
insert into t1 values (20 ,'Pb');
insert into t1 values (20 ,'Pc');
insert into t1 values (30 ,'Pa');
insert into t1 values (30 ,'Pc');
insert into t1 values (40 ,'Pc');

The least high tech approach (will also work on 9i)
SQL> select a,decode(min(decode(b,'Pa',1,'Pb',2,'Pc',3)),1,'Pa',2,'Pb',3,'Pc') B
  2  from   t1
  3  group by a;

         A B
---------- --
        10 Pa
        20 Pb
        30 Pa
        40 Pc

The Analytic function approach
SQL> select distinct a, first_value(b) over (partition by a order by decode(b,'Pa',1,'Pb',2,'Pc',3))
  2  from   t1;

         A FIRST_VALU
---------- ----------
        10 Pa
        20 Pb
        30 Pa
        40 Pc

The Obscure corner of aggregate function that can make experienced developers go 'WTF'
SQL> select a, min(b) keep (dense_rank first order by decode(b,'Pa',1,'Pb',2,'Pc',3))
  2  from   t1
  3  group by a;

         A MIN(B)KEEP
---------- ----------
        10 Pa
        20 Pb
        30 Pa
        40 Pc

SQL> select a, min(b)
  2  from t3
  3  group by a
  4  order by a
  5  /

         A MIN(B)
---------- ----------
        10 Pa
        20 Pb
        30 Pa
        40 Pc

SQL>