Withnail and I.
Reading through the thread many of the films listed aren't even close to cult films. Metropolis (cinema classic), Mad Max 2 (franchise movie, and not even the first in a franchise at that), The Thing (again I favourite of mine).
Monty Python and the Holy Grail, Jaws, Ghostbusters, The Terminator - all great films, but none of them are cult films. I love Fight Club, but it's not a cult film it's like saying Ther Matrix is a cult film, or calling GOT or Soparanos a 'cult' TV shows.
The Big Lebowski is a cult film, but Alien isn't a cult film, both are all-time faves of mine. Fear and Loathing in Las Vegas is a cult film, Repo Man is a cult film, Brazil is a cult film, Time Bandits is a cult film, Napoleon Dynamite is also. Boondock Saints? YES!