... in my humble opinion, is Trainspotting. You just can't beat the fact that English subtitles needed to be added to a film in English. We were introduced to the Edinburgh drug culture. The world met Ewan MacGregor (Rent Boy), Jonny Lee Miller (Sickboy and my personal favourite), and Robert Carlyle (the unforgettable Begbie).
My other favourites are:
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Inspired by a post on Cinematical.