[oracle] Oracle select most recent date record

I am trying to find the most recent record based on a date field. When I set latest = 1 in the where clause, I get an error. Please help if possible. DATE is a the field I'm sorting by. I have tried both latest = 1 and latest = '1'

SELECT 
STAFF_ID,
SITE_ID,
PAY_LEVEL,
ROW_NUMBER() OVER (PARTITION BY STAFF_ID ORDER BY DATE DESC) latest

 FROM OWNER.TABLE
WHERE   END_ENROLLMENT_DATE is null 
AND latest = 1

i think i'd try with MAX something like this:

SELECT staff_id, max( date ) from owner.table group by staff_id

then link in your other columns:

select staff_id, site_id, pay_level, latest
from owner.table, 
(   SELECT staff_id, max( date ) latest from owner.table group by staff_id ) m
where m.staff_id = staff_id
and m.latest = date

Assuming staff_id + date form a uk, this is another method:

SELECT STAFF_ID, SITE_ID, PAY_LEVEL
  FROM TABLE t
  WHERE END_ENROLLMENT_DATE is null
    AND DATE = (SELECT MAX(DATE)
                  FROM TABLE
                  WHERE staff_id = t.staff_id
                    AND DATE <= SYSDATE)

select *
from (select
  staff_id, site_id, pay_level, date, 
  rank() over (partition by staff_id order by date desc) r
  from owner.table
  where end_enrollment_date is null
)
where r = 1