Morphine (natural) is my favorite, with Oxycodone (synthetic) coming in second place.
I do love Diacetylmorphine, (Heroin/synthetic), however I have only snorted & smoked it (West Coast tar diluted in water/California - prefer snorting once I got the process down correctly), thus I have not experienced H to it's full potential; still, I enjoy it immensely and must place it here as my personal "wild card" at the moment, even though I am weary to IV as I know I would go crazy with it.