Another solution also using CROSS APPLY

SELECT MyTable1.x, MyTable2.y,..., G.Greatest

FROM MyTable1

JOIN MyTable2 ON MyTable1.key = MyTable2.key

.

.

CROSS APPLY (

SELECT MAX(T.v) AS Greatest

FROM ( VALUES (MyTable1.value), (MyTable2.value), ... ) AS T(v)

) AS G

Beer Molleman