First MDMA experience.
I'd done mushrooms before, but MDMA blew open the doors of euphoria and ecstasy. After that came LSD, 2C-T-2, 4-aco-DMT, andonandonandon...
2C-T-2 is reliably a wonderfully euphoric experience, and MDMA less so 7 years after my first time with it. MDMA has more/less become a simply euphoric stimulant, and that's still only using it once a year.
But as per thread title; I have yet to experience anything as euphoric as my first MDMA experience.
I think your most euphoric trip has less to do with the drug itself, but simply the fact that when things strike the synapses right, and you have no frame of reference in experiences like that; that's going to be the drug you view as being the most euphoric. Mushrooms didn't mesh with me, 6 hours of hell everytime, then MDMA, 6 hours of heaven. The beginning of my psychedelic using career was cemented, yet using 'purer' psychedelics such as LSD, although also giving wonderful experiences, didn't compare to the (MDMA) experience that started it all.
It's whichever drug blows open your doors of ecstasy for the first time that you'll view as most euphoric drug.