I've been reluctant to post here as I constantly use lines from films and tv, so for me it is VERY hard to choose!
When Harry Met Sally - definitely!
"Because of God."
My family has loads... most from Python films or Flying Circus, and most used when we get our communications confused, such as:
"I do wish you'd listen, Wymer. It's perfectly simple." (from Meaning of Life)
"Our two weapons are fear and surprise...and ruthless efficiency.... Our three weapons are fear, and surprise, and ruthless efficiency...and an almost fanatical devotion to the Pope.... Our four...no... I'll come in again." (Flying Circus, Spanish Inquisition)
"I told you once." (Flying Circus, Argument Sketch)
and loads of others.
On the Lord of the Rings extended versions, on the commentary track, Billy Boyd and Dominic Monaghan are hilarious, and coined another favorite quote:
"That's the way life is sometimes; you're just going about your business and someone throws away your lembas bread."
- Billy Boyd
(And I KNEW I had read a similar thread and posted that before!! There's another thread here:
http://talk.uk-yankee.com/index.php?topic=20452.0 - all quotes, not just movies)