having을 쓰세요.

아래와 같이 해보세요.

select *
from (
        select b, count(b) cnt
        from (
                select 'b' b from dual union all
                select 'b' from dual union all
                select 'a' from dual union all
                select 'c' from dual union all
                select 'a' from dual union all
                select 'b' from dual
             ) a
        group by b
     ) a
order by a.cnt desc
limit 1