For me it's Trainspotting. Not because of its accuracy but more because of the cultural commentary of the time. Thatcherite/post-Thatcherite Scotland, the heroin epidemic. Really the book is a million times better but the film still captures the essence in a surreal and really funny way. I haven't seen Requim or many others (I was put off of that one as I heard it was so miserable).
But yeah, visually Trainspotting is interesting and portrays an exaggerated version of the junkie lifestyle. It has an incredible soundtrack of course. But I love the humour which is rooted in the desperation of addiction, like going into the toilet to get the suppositories, with Brian Eno playing over it. Every desperate junkie has done a hit in some kind of disgusting environment. The withdrawals scene really resonates with me even though it is of course highly exaggerated also for effect. It works. But Trainspotting is about more than addiction - it is about Scotland after the industrial heart of the nation was ripped out and largely replaced by drugs. Still the OD capital of the world. The book tells the story in a much more intricate and Scottish way of course, but the film is a masterpiece. One of the best British films generally and a nice break from the usual bleak social realism that has generally defined British cinema over the years (with some significant exceptions).
No film can truly capture it though, nor a song, nor any artwork really. These things can resonate but it's kinda like pornography as an exaggerated portrayal of what sex might look like, but if you've never had sex, porn will never truly reach you what sex is like. It will only give you a portrayal of it. I don't suppose that needs to be said but I still think it's interesting to consider. Addiction is so visceral and all-encompassing. It is the most intense thing I've known and experienced next to sex and violence. Watching a film about boxing or watching boxing itself will show you how it looks but I it won't teach you how it feels to be punched in the face and knocked out. A film showing someone getting stabbed will never teach a person what it's like to get stabbed, just that it looks bad.
So back to Trainspotting, I think that the surreal nature of the film helps to capture the absurdity of addiction in a way that is fascinating and sometimes the absurd, the surreal and so on can tell the truth more accurately than "realism".
