well, this is only sort of what you asked... but i'm OBSESSED with heroin. i write about it constantly in my fiction and i'm always oddly eager to be a witness when someone's doing it (once i remember basically making my friend smoke heroin just so i could see it done -- don't get me wrong, she was going to do the heroin anyway, just wasn't going to smoke it).
but it's not that i've never had the chance, or can't get it. i've actually come so close to doing it that i had the straw in my nose, but i changed my mind, and honestly i'll never do it for a few reasons.
a) i am petrified of puking. i almost never puke and will go to great lengths not to, and heroin is known for causing vomiting. i've been told 9384678 times that puking on dope isn't bad at all, even if you really can't stand it normally, but still.
b) it's one of the very, very few things left on my "but i'll never do THAT" list. gotta keep SOMETHING on there.
c) no matter how good it actually is, NO reality can surpass my fantasies, whether it's about drugs or anything else.
d) i have an INCREDIBLY addictive personality. if i liked it and could get it with any regularity i'd be in trouble.
e) i'm more of a stimulant person. my DOC is probably adderall, with a minor in cocaine. sometimes i like taking some painkillers but that's a rarity.
other than that, my experimental days are pretty much over. there are very few drugs i've never done that i'd be willing to try now.
i guess i'd say desoxyn, because i LOVE stimulants and i've never done desoxyn or had it available to me, but because it's close enough in effect/etc. to drugs i have done (adderall, etc.) i'd still be willing to try it. but for example a few years ago, there were several drugs (mostly research chems and psychedelics) that i'd never done and desperately wanted to, but now, despite still being curious, i wouldn't do them. i just sort of outgrew that part of my 'drug career'.