1. Heroin, and by quite some distance. Simply the most euphoric, warm, blissfully apathetic high and nod. Especially IV, but in comparison to the other stronger ones, can also be sniffed or smoked to good effect.
2. Hydromorphone - only had this IV though, due to prohibitively poor BA via other routes. The rush is just as good as from heroin, but after the first 5-15 minutes it just lacks a little bit of legs and warmth compared to good ol' dope.
3. Morphine, IV - at an appropriate dose, doesn't fall too short of the first two. Also poor BA via other routes.
The best taken orally is strong poppy pod tea, which is mostly morphine but contains some other alkaloids that give it a more rounded effect profile, much more satisfying than just morphine taken orally.
I've tried just about everything there is to try except oxymorphone.