Fentanyl and its derivatives are generally agreed to be more addictive than most other opiates, but thats more because of the short duration of action and the fact that the extreme potency makes them very reinforcing (rapid development of tolerance, severe withdrawal syndrome etc), the actual buzz is generally reckoned not to be anything particularly special.
Hydromorphone and oxymorphone are very euphoric when IV'd, but orally they aren't nearly as good, probably due to slow uptake into the blood, limited bioavailability etc. Same with pethidine (meperidine), good when IV'd but crappy orally.
Dipipanone is often mentioned as being particularly good, but its only ever been sold in combination with cyclazine so its hard to say what it would be like by itself, as cyclazine is known to increase the euphoria of other opioids also.
For an orally administered drug, oral oxycodone is generally much more fun than oral morphine, but then the bioavailability is like 80% for oral oxycodone and 30% for oral morphine so that probably makes a big difference.
The one opioid drug that always seems to be mentioned as particularly euphoric regardless of route is ketobemidone, although I've never met anyone who's tried it, any Scandinavians on here?