I can second, third, fourth, and fifth what everyone else is mostly sayin, the most euphoric prescription opiate is (Drumroll, please): OPANA!
Just did some today (generics, not that tamper-proof bullshit) and I'm pretty damn euphoric. Could be that meth I smoked earlier in the am.
But I would like to try some Desoxyn now that I'm thinking about meth again. But fuck that when you can smoke that shit (I can't IV, nothing against it, I'd never come back and would be dead quickly).
But Opana beats oxycodone to shit, hydrocodone, and whatever else prescription wise you want to add in there.
I do like Heroin, but considering that's a street drug and I see very little of it, that obviously can't be included.