Euphoria is not a chemical released into your body, it's a subjective feeling which differs with each person. If you would ask a hundred people what they think is the most euphoric drug, you get a whole list of different substances, which clearly demostrates everyone has other opinions about the most euphoric drugs. For some it is psychedelics, for others it's opioids and for some MDMA or other stimulants. Some people find opioids crap because they give them little euphoria, while they love for example coke or amphetamines, and for somebody else it could be the other way around.
I personally find peaking on a high dose of psychedelics the most euphoric feeling there is, and it's a long one as well. MDMA comes close as well, but I lost the magic with it, but my first times with MDMA were pure bliss and heaven. I find opioids nice as well, but just relaxing and somewhat euphoric, but nowhere nearly as euphoric as a high dose of psychedelics, not even IV heroin or oxycodone.