I'm not sure actually, I've tried plenty, but I'd say its hard to say for sure because there are different types of euphoria and they can all get intense, I will list a few notable examples I've experienced during my various trials throughout the years.
Best rush goes to A-PHP + Heroin in the same rig, morphine + a-php woulda made me cum rainbows but was unavailable at the time.
Another was 5-APDB + Butylone, which I enjoyed more than MDA + 2C-B, was rubbing myself and moaning aloud in pleasure, something I had never before achieved, the best sex I've ever had couldnt compare to the euphoria, not that it beat the feeling of looking into my wife's eyes while simultaneously having an orgasm, but it came close... who am I kidding, it was as good, maybe better "I know I'm a dick but what evs" heh.
And most will disagree here but the best overall sedating high for me is codeine, its an old personal favorite that I don't indulge in often, get that warm hug like I get from heroin but stronger, I love it. 350mg to 450mg of codeine and 50mg of diazepam and your in for a good day.
My overall favorite class has gotta be them cannabinoids tho, nothing else can compare, PB-22 + JWH-210 in the same glass dick, the high, the intensity, the duration, pure perfection. Most people freak the fuck out from high dose noids tho, but I just get that noid nod that no other class of drugs has ever achieved for me. So I'd say that's the best combo I've ever done.