I've definitely been intoxicated on opiates, unable to walk without collapsing, slurring, nodding in and out mid sentence. This often occurred with morphine, heroin, and methadone, and without the aid of other sedatives. A few times I really remember it felt like my legs were sludge and it was so difficult to walk (heroin).
My first opioid experience ever was 30mg methadone and it nearly killed me (NOT SAFE). To this day I am surprised I did not end up in a coma. It was so awful for days, vomiting and spinning, unable to move really. I had no idea what I was doing. The classics newbie mistake... took 2 and didnt feel anything after an hour so took the 3rd, not knowing methadone peaks around 3 hours. Next thing I know I wake up (about 5 hours post dosing) and just start projectile vomiting. I've never been so intoxicated in my life. I was 13 years old and living with my mother.
More than likely the returns will diminish with most substances as you've said. I just dont think it's possible to categorize all substances or one as the most devastating. Of course, there tends to be some main players (opioids, alcohol, methamphetamine, cocaine). Though my boy Sigmund Freud was a huge cocaine addict! Lol it's interesting how many historical figures were into opiates and/or stimulants.
I believe dependency can look different, on different people, and with different drugs. For example, I was heavily addicted to black tar heroin and one day found myself smack dab in the middle of a huge methamphetamine ring; you ever see the movie "Spun"?
I used meth daily for about one year, in heavy doses via smoke and insufflation mainly, also oral. It's the one I didnt inject bc my girlfriend at the time wouldn't allow it lol even though we smoked crack and speed daily.
Anyway, meth was the easiest thing for me to walk away from. One day I left and haven't used it again and that was like 12 years ago. I was pleased to be away from it. I didnt break many of my personal morals for methamphetamine. However, this is clearly not the case for everyone.
Opiates on the other hand... they walked me hand in hand all the way to the gutter and nearly death. Every single line I set for myself, I eventually crossed. Every single one. I was incredibly dysfunctional until I had literally nothing. We each have our own demon(s) so to speak.
Side note... I was vacuuming the house yesterday and found a 30mg MS Contin stuck under the slider of a closet door lol. It had been there a while as much of the coating had worn away. Score