AMT is the most potent serotonin releaser by a wide margin. You can get significant euphoria from AMT from as little as 30mg - I actually have a friend who 30mg is a strong +++ and she compares the euphoria very favorably with 130mg of MDMA. My dose is 50mg and I'd compare it to 150mg of MDMA but AMT lasts 16 hours and you have euphoria throughout.
You are confusing euphoria with serotonin release though. There are a number of routes that drugs induce euphoria, the primary routes are through a combination of serotonin, norepinephrine, and dopamine release/reuptake. Pure serotonin release has been demonstrated. Its rather boring without adding something to release NE and/or DA as well.
Psychedelics function by stimulation of specific sites in the brain (usually 5-HT2A is considered the most significant) by binding with the site in a fashion that would be similar to the agent. In this case, serotonin. A really simplified way to look at it is that the receptors are locks made for a specific key (the neurotransmitter). The drugs that act as agonists of 5-HT2A (most tryptamine, phenethylamine, and ergoline psychedelics function this way) actually partially fit the receptor, activating it but jamming it at the same time so the effect continues beyond what the normal limit would be and also prevent the parent neurotransmitter from being able to reach the site. I'm pretty sure that I just greatly over-simplified that and that it doesn't always work that way either.
There really are way too many ways that drugs exert their effects to categorize them by their functions as releasing agents, reuptake inhibitors, agonists, antagonists, etc. It is a large combination of activities on various receptors that mediates the effects of psychoactive drugs.
If you look into the structure of most of them, they actually closely resemble neurotransmitters in the brain. Tryptamines are pretty much all structurally similar to serotonin (DMT, AMT and 5-MeO-AMT are great examples), phenethylamines are structurally related to dopamine (if you compare molecules of the 2C series and dopamine, for example, it is very apparent and also amphetamine to dopamine as well), LSD is actually both. It has the phenethylamine backbone as part of the tryptamine structure (LSD is part of a class of drugs called ergolines that share this characteristic).
Mescaline is literally dopamine with 3 methoxy groups replacing the 2 hydroxyls on the benzene ring.
Ketamine, PCP, DXM, etc., have completely different actions and can't be compared to the rest. They function primarily as NMDA antagonists.
Note: I am not 100% on this stuff. I never went to school for it, I never even took organic chemistry, this is just a hobby for me. There is a chance that I got some of this wrong and one of the brains from the Neuroscience and Pharmacology forum will come in here and fix it but I thought I would give it a shot
