Which is the most efficient way to run a particular select query?
I'd probably use

SELECT TOP (1) WITH TIES PolicyNumber, 
                         decpageid, 
                         Risk 
FROM   StatRiskDecpages 
WHERE  PolicyNumber = 'AR-0000301132-04' 
ORDER  BY decpageid DESC 


Assuming the covering index on (PolicyNumber, decpageid) INCLUDE(Risk) this will give you a plan like