Marijuana is hands down the best tasting drug out there. Fuck wine and brandy, cannabis is the ultimate of olfactory sensations. Good Cannabis can taste/smell like various kinds of citrus, blueberries, pine... I love a nice sweet tasting/smelling bud or wax.
Clonatril (banana flavored clonazepam/klonopin aka bananakins) taste fucking fantastic, literally like candy- a candy that melts away your worries. Yum. Although the original kpins taste pretty decent too- nice and minty.
Meth tastes absolutely horrible, almost like burnt sugar- even worse if you burn the pudd. Sometimes I get a real clean batch though, and it will have almost no taste.
Most pills don't bother me (probably the association, I sure did learn to relish the flavor of an OC, I could suck on those things like they were Jolly Ranchers) but Xanax/alprazolam tastes especially bad. Any Xanax tastes like the definition of bitter, doesn't matter what brand or dosage.
And then there's Heroin. We get tar where I live, and it's definitely not the best taste on the planet. I would describe it as sort of vinegary, almost like a burnt barbeque sauce. Definitely not terrible; I certainly learned to love the flavor. Even now, years since my last dose, I can still taste it. Mmm, mmm, good.
So yeah, I'm goin with Meth as the worst tasting drug out there.