Morphine is the strongest opiate by definition, as the most potent alkaloid present in opium. Once you start altering or adding to the chemical structure you've no longer got an opiate but an opioid. That said, heroin is a semi-synthetic opioid and is more potent because the two acetyl groups allow the molecule to more easily penetrate the blood brain barrier.
The fun doesn't stop there though, there a number of opioids far more potent than heroin.
To give you a general idea they are as follows:
Heroin - roughly twice as potent as morphine.
Methadone - Said to be 3 times as potent (oral) as morphine, also has a much longer half life.
Hydromorphone - At least 5 times as potent as morphine.
Oxymorphone - about 8-9 times more potent than morphine.
---------Here we go from semi to totally synthetic----------
Fentanyl - 80 times more potent than morphine.
Sufentanil - approx. 800 times more potent than morphine, 10 times more than fentanyl.
Carfentanyl - A whopping 10,000 times more potent than morphine.

There are more opioids of course, they vary not only in potency but also duration (some lasting only several minutes), cardiovascular and respiratory affects.