Luckily, in the past I had truly unlimited access to any and all pharmaceutical opiates (brand-name) used/sold/produced in the United States, and at a low cost as well. Needless to say, I've had more than my fair share in tasting every single one. I'll just cover my personal opinions regarding the "big" ones, in small paragraphs.
Codeine, which I obtained in 60mg IR tabs (only ingredient was Codeine, no APAP), really just makes me itchy and uncomfortable now. When I was opiate-naive, I remember taking 8 Tylenol3's and feeling amazing. I even caught a nod! It was the first time I had ever "nodded" off of opiates, and it was also what got me interested in the first place. It gives me a HORRIBLE histamine reaction, though, so I don't take it, no matter what. I list it as a "Patient Allergy" anytime I go to the doctor/E.R.
Hydrocodone, obtained in all forms but primarily Norco 10/325mg, made me nauseous the first time I tried to get high (15 or 20mg), but after the nausea and cold-sweats passed, I just laid on my couch and was content with everything and doing nothing. As my tolerance went up, and I had to CWE, I found that doses higher than 100-120mg produced extreme empathogenic effects. It was similar to X, but obviously an opiate. I was sedated, but very talkative and empathetic. It's okay, but it doesn't work for pain for me.
Oxycodone, obtained in all forms but primarily Roxi30's and Oxy 40/80's, is a very stimulating opiate for me. I would snort/shoot an 80 and then go clean the entire house and vacuum and all that. If I increased my dose, it would cause sedation. Currently I am Rx'd it in the form of OxyContin 20mg's, for L5-S1 rupture and herniation, and it does jack-all for killing pain for me. I WILL be asking to be switched to a -morphone (i.e. Opana) as I am currently unable to walk, have a job, or go to school. I do feel that Oxycodone is one of the most over-rated opiates.
Hydromorphone, obtained in the form of Dilaudid 4mg and Hydromorphone 2mg/mL ampules, was the first opiate I ever IV'd (I found an amp and rejoiced!). It hit me hard, and the feeling was like nothing I'd ever felt before. No doubt this is what triggered my addiction. I would say that, euphorically, the rush is second only to oxymorphone, although the "high" doesn't last much longer than 2 hours, if that. Very little histamine reaction, but the drug is worthless in any ROA besides IV. I used to mix up, say, 24mg Dilaudid with around 30mg Opana, fit it into one needle, and shoot it. Almost OD'd once, but it was incredibly, unbelievably, the most euphoric thing I have ever experienced.
Morphine, obtained as MS-IR 30mg, MS-Contin 200mg, Avinza, Kadian, and MS04 10mg/mL ampules, is great until your tolerance really gets up. I was having to IV nearly 700mg of Morphine at once to get a rush and maintain a high, but it caused such a strong histamine reaction that I stopped using it all-together. At that dose, my face would turn bright red and feel like it was on fire, the pins and needles would hurt, and I'd get a rash along the vein I injected into. Other ROA's proved to be less-than-exciting, so I just stuck with the "better" stuff.
Oxymorphone, obtained as Opana ER 40mg and Opana IR 10mg, is BY FAR my favorite opiate/opioid. It's analgesic properties are second to none, regardless of ROA, and same for euphoria, except the euphoria is only truly obtained via IV usage. I got my tolerance to upwards of 70mg a day IV. I would also snort the 40mg ER's, which actually kept me high longer than any other opiate I've snorted (6 or so hours, with a HUGE tolerance). I mentioned my current injury above, and it's got me at a 7 or so on the pain scale, peaking at a 9 or 10 (yes, truly the worst pain I've ever felt; radiates from my back down to my knee and makes my leg useless; I have to use a cane to walk), and I really am going to try to get my doctor to switch me from OC and Percocet to Opana ER and IR; not for abuse, but for legitimate pain treatment. He seems not bad so far, Rx'd OC's on second visit, and with Endo touting it as the "new, less-addictive, un-abusable alternative to OxyContin", I think that it's a good possibility. I'm a Pharm Tech, so I'll just mention about how our distributor kept pushing the stuff on us, trying to get us to switch patients to it from OC. I've got my fingers crossed.
Heroin, obtained via street, is something I've used only because my access to these pharmaceuticals was terminated (I moved to another state). I've never gotten a true "rush" off of dope, and had a habit of a gram or so a day, but I used it to keep me well. The euphoria was decent, but not much more than morphine's. This was only used IV, and being that it's street dope, that means purity is up in the air. Based on that, I can't give a true, accurate statement as to comparable effects.
Fentantyl, obtained as Duragesic, Actiq, and Fentanyl Powder, has always been super-heavy on the Nod, but weak in the euphoria and how long it lasts. It always gives me horrible heartburn, and for that reason I don't seek it out. When it comes around, I don't turn it down, either, though. It's decent at analgesia, but I wouldn't want to be Rx'd the patches, as most people don't have much good to say about them, and I only ever used them for the gel.
So, apparently I'm a huge fan of the semi-synthetics, don't much care for synthetic or natural.