Like others have said, everyone is different - I think it's a bit odd of you to make the assumption that H is the most overrated opioid there is - perhaps for you, but I'm pretty sure the many many users of it would disagree with you. Well, there are certainly a few, the ones in this thread for example
Agreed that its negative effects are numerous, and it's hard to strike a balance between those and the positive ones, the respiratory depression being of the biggest concern. I only experienced nausea the first time I did it, even in large amounts, and I've never had hiccups or dizziness. It can make a lot of people feel nauseated, yeah, but I personally find Fentanyl and its analogues (like parafluorobutyrfentanyl (PFBF)) to make me feel the most sick out of any of them. I'm quite prone to opioid headaches (bupe giving me the worst headaches I've ever had in my life, codeine headaches are quite bad too, plus nowadays it's not even worth bothering with) and heroin has never once given me one. H makes me pretty itchy, but Codeine is by far the worst for that, and it sucks.
There are shitty side effects associated with all of them, and it depends on personal chemistry on which opioids will give you the most, and the worst. One side effect I feel a lot is anger/irritation - I get pissed off and snappy with people around me really easily, and this applies to heroin, Fentanyl, AH-7921 (probably the worst for it, and it wasn't that good either), bupe, and I'm sure some others. This doesn't really matter most of the time though, I usually save opioids for when I'm alone, I don't find nodding out a brilliant way to socialise, except with my smack buddies, who I don't see that often.
H is extremely euphoric for a lotta people that have used it - obviously, because there are so many addicts. I don't think it's entirely down to availability, though that will be a big influence in most places, there are a lot of places where getting pharmaceuticals is not that hard and is also quite cheap. Of course, there are plenty of pill addicts, though I would say most pill poppers are more hidden than H users. It'd be hard to compare numbers, though I'm sure H would come out on top, a lot of people that can afford to be addicted to pharms probably aren't on most measurable addiction statistics, I would assume. I could be wrong - except the online, survey-type ones I guess. Plus, like TheLostBoys said, it may well be cheaper at the start (though it probably depends where you live and what you're buying from whom) but tolerance rises fast, and often I've seen Fentanyl for dirt cheap, even as low as half the price of smack dose-for-dose (not mg for mg of course).
Some people do enjoy the nod, and some people don't, and the H nod is an extremely dreamlike, powerful one. Codeine & Dihydrocodeine are pretty much impossible to nod on imho, even when I had lower tolerance, Oxycodone is a nice one but not very sedating. Sometimes that's what I like, cause nodding off when you're trying to get shit done or even just play games or read a book is annoying, sometimes though I wanna nod the fuck out and getting to that point with Oxy is really expensive. Fentanyl is pretty much just falling asleep for me, without a great deal of euphoria.
One of the best nods, probably struggling for the top spot with H in my book is that of a heavy dose of poppy pod tea - which might sound silly, but it is really bloody nice. An annoying side effect of PPT though is just feeling full of liquid - I probably could experiment more with making putty to solve that problem though.
I don't inject (I'm a pussy, and I know right now it would rapidly become really problematic for me), I only snort opis - with the exception of H which I snort and smoke, and Oxycodone which I mainly do orally cause I found snorting it to be a waste by comparison (I still get it in the form of capsules though, cba with all that filler in case I do fancy sniffing a bit of my dose for a quick buzz before the oral part kicks in). My favourite used to be Oxy, for a long time, cause it was the first of the "big boys" I tried - now that I've had H and poppy pods though, I think it's been knocked off the top spot. I still love oxy, but I'd probably take a fat line or two of smack over an OC80 nowadays.
From what I've heard, sweet heroin is usually cut - it could be Fentanyl, it could just be H - with some kind of artificial sweetener like mannitol or sorbitol perhaps. Doesn't mean it's not good, but it might not be, obviously it could just be that you don't rate heroin highly anyway, but if it's cut that may well be why it makes you feel kinda shitty, whether it's good or not. I can't imagine smoking or IVing artificial sweetener is particularly pleasant. That may also contribute to why you're of the opinion that a lot of smack is really stepped on (well, almost all of it is, that's true, it just depends on your connects on how much yours is). All the good H I've had has been off-white/tan/brown and has always tasted pretty nasty - like vinegar, fish (is this just me?) and poppies when chased. All I know of what I get's purity is that it only takes between 10-30mg to get a good nod on.
The ones I think are most overrated are Fentanyl, Codeine, Dihydrocodeine and Tramadol.
Fentanyl cause it just puts me to sleep, fuck all euphoria really, highly addictive and shit to withdraw from.
Codeine because it has a pretty low ceiling dose when you're tolerant, rappers and shit make it sound like it's the bees knees but it's just massive histamine release for no nod, and not a great deal of pain relief when you've moved further up the opioid ladder, and very little euphoria. It's also pretty likely to give me a fat ass headache. Similar with Dihydro but without the ceiling dose thing.
Tramadol because it makes me feel all weird and uncomfortable, and I'd reach the seizure threshold way before I would nod or get much euphoria (even before I had tried many different opioids) plus the serotonergic properties of it make me feel pretty depressed after stopping using it.
My favourites are probably heroin, poppy pods and oxycodone, in that order, though H and pods may switch round depending on my mood. I do also really like Parafluorobutyrfentanyl (PFBF) - even though it can make me feel sick for the first couple of doses, it's different enough from its parent that I get quite a euphoric, dream-like nod from it, and most of the other side effects aren't too bad - with the exception of the irritation, but that's not a problem if I'm alone.
H, pods and PFBF also have the added pleasure of giving me some pretty cool visual hallucinations, just like squiggly lines, symbols, numbers, and sometimes piles of clothes or shadows can look like my friends
PFBF is additionally cool because when I start to nod, I get some auditory hallucinations too - imaginary conversations, and sometimes little pieces of music or jingles. It's really very nice like that.
I'm not a great fan of buprenorphine, but I wouldn't say it was overrated because obviously it's really useful for people coming off other opioids, and I'd use it if I was coming off something too, I'd just have to take an NSAID and some paracetamol for the headaches.
Like I said, everyone's personal chemistry is just that - personal - and it will greatly influence what kind of high they like, e.g. I don't always aim for the nod, sometimes I like to be high and function. It'll also influence what kind of desirable and undesirable effects people get, so that'll influence their preferences. When I first tried smack last summer, I thought it was overrated too. But then I had it more times and it grew on me - I have no idea how much experience you have with it, and obviously I'm not suggesting this as something to try out! I do wish I'd have left it alone, because even though I'm not dependent (yet...) it has made me crave all kinds of opiates and opioids way more than I ever did before. Previously I'd do them maybe 2x a month, tops. Nowadays I do them a few times every week. H gave me the first feeling of crazy, don't-give-a-fuck bliss, where I'd only experienced sleepy nods or mild euphoria before (maybe I'm being a little harsh on oxy, I did nod on it, I just usually preferred to use it for functioning, medicating my chronic pain)
Anyway, these are just my opinions, everyone's is gonna be different really. I guess that's the point I'm trying to make, but I'm kinda wasted so I've just gone kinda off-topic and rambled again

also, if I sounded like a dick or patronising or condescending at any point, that wasn't my intention, I'm just contributing to the debate (I presumed that's what you were inviting folks to do) and giving my opinions on various other opiates/opioids and stuff. I'm not good at recognising how I sound to others in my posts, so please don't think I'm trying to offend anyone or be a douche

Peace,
RL