Hey gusy, so unfrortunately, this isn't really a Harm Reduction issue that we're dealing with, so I'm going to move it over to Drug Culture. Like others have said, I don't really think that they are comparable in a real "apples to apples" kind of way. As
@Las Veghost grower has said, it is a highy subjective thing. We're also dealing with thoughts and emotions that are extremely visceral and elusive to description with words and if one person can descfibe it "accurately" the next person might not understand it "accurately".
So, while I think that a lot of the feelings can be compared, I don't think it can really be done in the way that you're asking, especially when you ask questions like, "what is the most euphoric of all". That's just not possible to do.
I think the euphoria of Heroin or Opioids in general, is, at the end of the day, a more "numbing" experience than anything. William Burroughs said it in a way that I have always enjoyed in that, "Heroin is a drug that narrows". He mentions this in the context of going to the Amazon to find a drug that "expands". He is ultimately looking for Ayahuasca. In the book, it's referred to as "Yage" and it has a semi-mythical status in the author's mind.
Some people will say that they have a just orgasmic euphoria from taking a high dose of LSD or some other hallucinogen. That's definitely possible, but those drugs don't "do the work for you". MDMA is a drug that "expands", I would have to say. It can bring people together and cause people to access and/or express their emotions in a more true way than they might be able to without it.
It's all very subjective and I believe life experience and biology all play roles as variables in what leads a person to develop a preference for one over the other. I'm like
@Las Veghost grower I have always been drawn to Opioids. And like her, I'm sure I can enjoy stimulants too, but they're just not my preference.
To Drug Culture we go.