I registered here to reply to this post since no one has mentioned the opiates stronger than heroin.
Fentanyl is a synthetic opioid that can be up to 100x more potent than heroin and the ld50 is 0.5mg-1mg. I first ran into fentanyl back in my morphine days. Nobody had any morphine, but a guy said he could get fentanyl patches which were, he said, a substitute for morphine. That was all I needed to hear. I had done morphine every day, sometimes in amounts exceeding 250-350mg, for over a year. I still managed to work and paint, but it got to the point that I couldn't be happy without having morphine. So after being out for little over a day, the desperation began to set in. Anyone who has been addicted to an opiate knows that kind of craving. Unable to find my drug of choice, I bought 10 25mg fentanyl patches. It was a gift in disguise. I don't remember how many I used, but I remember pulling over at a gas station and wobbling into the bathroom. My skin was cold and clammy and I just laid against the bathroom wall, unable to move. I was higher than I had ever been in my life. The tiles and stalls and toilets started to take on a glowing quality. They almost looked holy. Anything strong enough to make a convenience store restroom look majestic is pretty powerful. I don't remember anything between passing out and waking up in the hospital. I have been clean (from opiates) ever since.
I always preferred morphine to heroin, and maybe I'm alone in that, but I had done heroin more than a couple of times. Compared to fentanyl, heroin is tylenol.
Stronger still is an analogue of fentanyl, Carfentanil.
Carfentanil is roughly 10,000x more potent than morphine, 4,000x more potent than heroin, and 100x more potent than fentanyl. In terms of raw power, Carfentanil is probably the most powerful drug. It's lowest median effective dose is 0.00032 mg.
So unless you're a wild animal weighing in at over 1,000lbs, I'd stay away from it.
The strongest hallucinogen I've ever experienced is brugsmansia. At the time of my first experience with brugsmansia I had done LSD, LSA, mescaline (my favorite), ayahuascha, all manners of mushrooms, DXM, ketamine, salvia, datura, etc ... more than I should have in truth. I've had scary experiences with these drugs, and sublime experiences as well. DXM and salvia can be terrifying, especially with someone prone to anxiety and panic attacks (such as myself) but the pure terror of brugsmansia surpasses them all. At least for me, anyway. I was not even seeing the same world my friends were seeing. With LSD there are perceptual distortions (breathing walls, glowing colors, etc) but with brugsmansia all of my senses were feeding my brain incorrect information. In reality, I was spazzing out on a couch. In that other world, I was playing hide and seek with some sort of creature. It never gave me enough time to hide so I was always running. Anxiety nightmare. Possible seizure. Terrible. LSD is pretty tame by comparison. Again, this is personal experience and might differ greatly from person to person.
Hope this helps.