Oxymorphone + cocaine, in the same rig and injected straight into a vein. I'm not alone in thinking the rush from IV oxymorphone is better than heroin, so that makes the combo better too.
There's simply no comparison to an IV speedball of a strong opiate and coke. It's just pure physical pleasure that can't be matched by anything. Not even your best orgasm comes close to that rush (emotional fulfillment from love is something entirely different than physical pleasure). And you can't even compare snorting the two, because the rush from shooting it is orders of magnitude more pleasurable than just snorting them. Some people judge a speedball from their experience snorting some coke while high on opiates, and conclude it's not the be all end all of physical pleasure; these people have no idea how huge a difference IV is.