My late mum and I didn't always agree on what constituted our favourite movies--but we had about a half-dozen we did agree on, including: The New Yorkers, Galaxy Quest, My Fellow Americans, Saving Grace, Rat Race, (mum was a huge comedy fan), Plus, we frequently watched Under Siege (I) and All the President's Men...and, mum's all-time favourite movie, is also one of mine: Arsenic and Old Lace. A movie that I truly think can be labeled a "timeless classic."
Here's one clip from the movie (yes, those bad guys are who you think they are--note the "in-joke" quip at the end of the clip):
