Get Root parent of child in Hierarchical table

DECLARE @id INT = 6
;WITH parent AS
(
    SELECT id, parentId, 1 AS [level] from tbl WHERE id = @id
    UNION ALL 
    SELECT t.id, t.parentId, [level] + 1 FROM parent
    INNER JOIN tbl t ON t.id =  parent.parentid
)
SELECT TOP 1 id FROM parent ORDER BY [level] DESC

@TechDo's answer assumes the lowest ID will be the parent. If you don't want to rely on this then the above query will sort by the depth.

Please try:

declare @id int=6
;WITH parent AS
(
    SELECT id, parentId  from tbl WHERE id = @id
    UNION ALL 
    SELECT t.id, t.parentId FROM parent
    INNER JOIN tbl t ON t.id =  parent.parentid
)

SELECT TOP 1 id FROM  parent
order by id asc

You can try this query my friend to get all ids:

with tab1(ID,Parent_ID) as
(select * from table1 where id = 6
union all
select t1.* from table1 t1,tab1 
where tab1.Parent_ID = t1.ID)
select ID from tab1;

and this query will give the final result:

with tab1(ID,Parent_ID) as
(select * from table1 where id = 6
union all
select t1.* from table1 t1,tab1 
where tab1.Parent_ID = t1.ID)
select ID from tab1 where parent_id is null;

SQL Fiddle