Meth had the strongest and most intense euphoria, maybe aside from MDMA, but MDMA's other effects are quite intense too. With meth I was able to enjoy the body rush much more than with MDMA because it didn't make me start rolling (obviously), so I basically only felt the 'full body orgasmic' feeling and the stimulation. MDMA offers those effects as well, but is more distracting because it also makes you 'kind of' trip (nothing like LSD, but there's a lot of distortion in my visual field on MDMA), therefore I think meth's euphoria was stronger and 'cleaner' than that of MDMA.
So, in short; Meth produces euphoria that is quite a bit stronger than the euphoria opiates give me. BUT, the reason I still prefer opiates BY FAR, is meth's many side effects. After a while that initial euphoria wears off, but you remain stimulated as fuck for a long time. During that time, the stimulation can even become annoying at some point. Because it will still be very present, and you will be wired as fuck. But the actual euphoria will be gone by that point. And redosing just doesn't work that well with uppers IMO. Every time I redose I start noticing some more side-effects that I would also get if you just didn't redose and came back down immediately instead of redosing.. But each time I decide to redose anyway those side-effects become stronger and more annoying.
That's why I prefer opiates' euphoria by FAR. You can keep an opiate high going as long as you have opiates, there is no come-down or crash after a single use. Also, opiate's euphoria, while less intense, is far more mellow and relaxed. I like feeling mellow and relaxed. With stimulants I always feel like I get a high, and then I have to pay for getting high by feeling shitty as hell. The longer I keep the high going, the shittier I'll feel. With opiates, there is never a comedown. Of course opiates are physically addictive, which is its own kind of hell. But at least you don't have to go through a period of feeling like total crap every time you decide to dose up, and I do get that from stimulants.
So, when we're strictly speaking about the strength of the euphoria I'd have to say meth is the winner,
but (by far) my favorite kind of euphoria is opiates'.