Withnail and I (1986)
They say that Withnail and I is a cult classic and I can see why. It's the type of movie that you see and shrug off, as though it has no imprint on your mind. And then the next day you are trying badly to quote it to people who will just stare at you blankly while you try to explain "It's from this movie..." This will go on for quite a while until one of your friends see it and suddenly every time you are having a drink there's this new game where you try and see who can keep up the banter from the film going the longest. Of course, everyone else will still just be staring at you and passing glances to one another, but at least you are not alone.
When I saw the guy that was the old British roadie in Wayne's World 2 in this movie doing the exact same character and realized that he had been reprising a role all along...it blew my mind.
In addition to funny lines it had great dreary atmosphere and the most despairingly beautiful set design. If you like British humor at all or alcoholics in general you should have seen this by now.