Alright, so heroin and hydromorphone are way better than oxycodone - I totally agree. That's one of the reason I preferred to lose a lot of money on the beige shit than shoot up lab grade oxycodone when my morphine source disappeared.
But it wasn't for the rush, it was for the general feeling. And that's why for me there's an opioid way better than heroin or hydromorphone. It's morphine. And it doesn't matter it crosses BBB less easily, let's say you get X % of heroin into your brain where it's metabolized fast to morphine and 6-MAM - what you feel is just an instant high for your mind, head, whatever you call it. Now with morphine you might get even X/2 % of it into your brain, but you get both the feeling in your head and there's the great bodyload.
Certainly you may not like the "pins & needles" feeling AND some may not like feeling like being hit with a hammer in their heads - that's what's heroin for me, very little bodyhigh compared to morphine. And concerning bodyhigh and completely no rush (the way you described it) levorphanol beats all of the drugs mentioned in this thread, really. And it goes on and on...
If it was only for the difference between the time you hit the vein and the time when you feel the strong rush, drugs like fentanyl would take place of heroin on the street due to the easiness of synthesis long time ago. And still, when there's a batch of heroin with fentanyl mixed in, most people don't want such a combo. The reason why people choose heroin is generally the fact that there is actually no other opioid on the market that could match it, there is no choice. Methadone is good for long, lone, and sedative parties, buprenorphine is good for beginners (8mg tablet is enough for 8 opioid-naive ones), fentanyl is dull in its selectivity and after you inject it, the next thing you know is you have to repeat it... Codeine and dihydrocodeine are good and I mean it because I've been there long long time ago and loved it but wanted more and got it... What's left in the pharmacies? Hydrocodone, oxycodone, hydromorphone, and oxymorphone in some countries. The first two aren't really worth mentioning as they're a lot worst than morphine when you compare them intravenously. Hydromorphone is a still good one and the pills with it contain relatively low amount of additives so it's easy to get a solution for i.v. But oxymorphone is rarely prescribed in the form of IR pills so it's usable actually only in the form of lab grade powder.
I don't know anything about getting oxycodone out of ER pills, but now that I'm mentioning that there's no choice, I'm seeing this one might be one for those who either don't have access to heroin or heroin costs them more than these pills. Anyway, some 3 years ago when I still shot up and I had contact with people "from the street", I often heard them asking me for morphine as they knew what I had shot up earlier. Any addict praises pure morphine from an ampule over brown powder with at most 10% of heroin freebase. It might be even heroin hydrochloride but let's be honest here - who likes adulterants?
Oh, and last thing, I've seen quite a lot of studies done to see if addicts can tell apart morphine and heroin when they aren't told what they're given. And the results showed in various ways that it was impossible to tell which samples were what. Besides even though heroin is used in medicine as a last resort analgesic, e.g. in the UK, I don't really think people who have morphine or something else switched to heroin would feel the difference if there wasn't this stigma put on heroin. On the streets it helps to sell it to opioid addicts (who actually don't care if it's heroin anymore, they just want the fix), in medicine it helps to convince patients it will help them better.