so far, for me, it's IV Heroin. i register several times while injecting, just to make sure that my shakey ass hands haven't pushed the pin through the vein. by the time half of my .5 or .6g shot is in me, and i'm drawing back blood, and i start to feel the warmth...there just isn't much else that even comes close. i iv'd 450mg of oxy this weekend, and it didn't come close either. there's just something about heroin.
i would say that mdma is very euphoric, but euphoria to me means being able to communicate and stay in control of myself. mdma tends to send me, which isn't a bad thing at all, but it is what keeps mdma from being the most euphoric drug imo.
i don't even have a second place. opiates/opioids in general occupy the top rung of the euphoria ladder. mdma, coke, alcohol, amps, etc all have their place...but, IV'd heroin is, really, far and away the most euphoric drug to me.
as a disclaimer; i shoot enough to kill most people, and i tend to get pretty damn good tar (hard, and crumbles into a rocky/powdery substance). i'm not really sure that it would beat out the likes of Oxys at lower doses.