This is no doubt almost purely subjective...
Strong dose pure MDMA - most intense single drug - induced (as opposed to multiple compounds taken simultaneously) euphoria
QUALITY cocaine ( As in what remains following extraction such as Le Junk presented ) excellent euphoria, much cleaner and less imposing than even the best meth
Strong dose clean mephedrone - closest thing to MDMA feeling, though not as intense of euphoria. If I were to rate strictly the euphoria I would probably rate this as equivalent or even superior to all but the absolute best of coke, but to me that coke euphoria is pretty damn amazing...not to mention that with mephedrone there seems to be an almost sinister undercurrent on the back end. Tough to pin down in scientific terms, but there nonetheless.
QUALITY meth - great euphoria, but not worth additional the ten hours of stimulant static.
Quality sativa - Enough said
The above represents the most common result. That being said, I find that in order to obtain the most intense euphoria, one must include a good clean psychedelic, the best combinations being those like LSD/MDMA or similar. The psychedelic induced euphoria is singular in its intensity - as in life changing/religious epiphany type of thing, as opposed to the more physical/humanist hedonistic type of eupphoria most classically induced by good cocaine. I prefer the combo, seeing as they tend to be synergistic, and most especially since even the best cocaine euphoria pales in comparison to the psychedilic/empathic euphoria, primarily because of the perceived relevance and significance.