Sorry, guys, you're all wrong. I've seen 90%+ of all the movies mentioned above, and while some of them comes close (Holy Grail, Big Lebowski, Clerks), they are easily beat by the English black comedy & underground cult favourite "Withnail & I".
Movie quotes are seldom very funny out of context, but for those who've seen it, a quick recap if you wish:
http://en.wikiquote.org/wiki/Withnail_and_I