Okay, so this post on the different highs is not going to be subjective. I am going to report what I have gathered from others' reports on the differences in opiates, because my report would be different, as all reports are different. So I am attempting to put together a collective report of the differences in opiate highs.
Oxycodone- Oxycodone is usually regarded as being quite a euphoric opiate, and often considered quite stimulating compared to others. It is reported to last a bit longer than hydrocodone, but not substantially, and not nearly as long as morphine, for example.
Hydrocodone- Hydrocodone is usually regarded as an opiate slightly inferior in effects to oxycodone, however, there is a portion of the opiate-using populating who report hydrocodone to be especially enjoyable (more so than oxycodone) for reasons they cannot exactly articulate. Hydrocodone is usually considered less stimulating than oxycodone however this differs from user to user, and some consider hydrocodone to be quite a bit more stimulating than oxycodone. "Dreamy" is a word I have heard used more than once to describe the effects of hydrocodone. It seems users enjoy combining carisoprodol (Soma) with hydrocodone as there is an apparent synergy, however, while certainly existent the combination of carisoprodol and other opiates seems to be less reported. Often users will report enjoying using hydrocodone in the form of Tussionex especially, possibly due to the fact that Tussionex contains a sedating anti-histamine to potentiate the effects of the hydrocodone, and increase the nod.
Heroin/Morphine- I have combined these two due to the quite similar effects with some notable differences. Firstly, the rush of IV heroin is regarded to be far superior to the rush of IV morphine by a good number of users. The IV heroin rush is said to be one of the best opiate rushes often compared in intensity to hydromorphone's rush IV, however, some will say it is more intense, others will say it is less intense. IV morphine is said cause especially strong feelings of "pins and needles" due to a great deal of histamine being released throughout the entire body. After the rushes, the effects are said to differ, and heroin's effects are generally favored over morphine's, however, this varies. Both heroin and morphine, regardless of ROA, are said to be more sedating, heavier opiates than either oxycodone or hydrocodone. They are said to be much noddier. Some opiate-naive individuals may not like the effects of morphine as much as oxycodone and hydrocodone because morphine is more subtle in effect, and seasoned users are more likely to appreciate this.
Codeine- Codeine is metabolized into morphine and another metabolite(s)(?), both of which cause the effects of codeine. Some people cannot metabolize codeine properly because of biological differences, and so cannot enjoy codeine at all. Others can only metabolize poorly, and some can metabolize well. Codeine, however, is generally regarded as a "lesser" opiate. I suspect the cause for this is due to the fact that like morphine, codeine is more sedating and heavier at an equipotent dose, with less apparent euphoria, and seasoned users, who would appreciate such effects, would require more codeine than could be taken (due to ceiling dose) to have it hit them good, and it is simply not feasible. Some have reported very good experiences with codeine, though.
Fentanyl- Fentanyl is generally considered quite sedating and quite noddy. Some report it does not have as much euphoria as other opiates, but some like it very much. It is very strong, nonetheless, in its effects. Its effects, however, are not long-lived.
Buprenorphine- Most users of buprenorphine are opiate addicts, so this should be considered. Buperenorphine is often considered to be quite stimulating in its effects, however, the effect of buprenorphine largely depends on the dose and the state of the opiate addiction (wd's, maintenance etc.). It is quite long lasting, longer than morphine or heroin by general concensus.
Methadone- One thing most users will agree on is the strength of methadone, its sedating qualities, and its very long lasting effects. It is a very potent mu-agonist and can dull other opioids taken while taking methadone. Some regard it as quite euphoric, others do not seem to see such euphoria in the drug. A good deal of side-effects are reported with methadone compared to other opioids.
Butorphanol- Butorphanol's impression on people varies quite a bit. Some love its effects, and find it to be exquisitely euphoric (often when they have had it in clinical settings), and some report side-effects such as restlessness and nausea. It is a partial antagonist, and has some dissosciative properties possibly adding to the varied responses.
Hydromorphone- Most regard the rush of hydromorphone IV as wonderfully powerful and euphoric. Hydromorphone is not very long-lasting in effect, perhaps about as long as hydrocodone or slightly shorter based on others' reports. The rush that the drug provides is often considered its main asset. It has been described as so powerful that it often creates a bit of anxiety when occuring, that it is too large a dose. It's high after the rush is said to be quite nice, comparable perhaps in enjoyment factor to hydrocodone or oxycodone.
Well there it is, a crude, roughly put together summary of a few opioids which I have read and heard a lot about via individual reports. Again, this is really rough, and I am sure not very inclusive of a lot of these drugs' other properties.