Human Traffic or Candy, and Fear & Loathing gets runner up for sheer weirdness
Those first two are the only ones that I felt really captured the experience of their respective drugs, that managed to convey the feeling the drug provides or depict in convincing terms the relationship the user develops with the drug. I think personal experience is a big part of it, Candy is based on an autobiographical book, and anybody who's seen Human Traffic can tell you that the guys who made that film have obviously had plenty of nights getting off their nut in the club scene, then of course there's Hunter S. Thompson...
Films like Requiem for a Dream come off as overblown and understated in all the wrong places because the people who made them obviously gain their drug knowledge second hand.