Taking a shot at it:
SELECT DISTINCT
patid,
fillDate,
scriptEndDate,
MAX(DATEDIFF(day, fillDate, scriptEndDate)) AS longestScript,
drugClass,
MAX(rn) OVER(PARTITION BY patid, fillDate, drugClass) as distinctFamilies
FROM (
SELECT patid, fillDate, scriptEndDate, drugClass,rx.drugName,
DENSE_RANK() OVER(PARTITION BY patid, fillDate, drugClass ORDER BY drugName) as rn
FROM [I 3 SCI control].dbo.rx
WHERE drugClass IN ('h3a','h6h','h4b','h2f','h2s','j7c','h2e')
)x
GROUP BY x.patid, x.fillDate, x.scriptEndDate,x.drugName,x.drugClass,x.rn
ORDER BY distinctFamilies DESC
Not sure if DISTINCT is really necessary - left it in since you've used it.