Cannabis. Great, potent, seedless, to come in different flavors and strains for different occasions. It means I would stop wasting money and have a great anti-depressant for the rest of my life with minimal side effects in comparison with most of the drugs.
On the other thought... Getting something good in unlimited qualities would probably mean a failure to self control at some point and a greatly reduced life longevity. But for heroin users out here I can see how AWESOME it would be to get pure pure shit and just use it daily and have normal lives. Ahh, dreams.