I can't limit myself to one.
Pride & Prejudice with Colin Firth
Room With a View
Persuasion--the one with Amanda Root
Emma--Gwyneth Paltrow as Emma
The Way We Live Now (it was on Masterpiece Theater a few months ago and my friend taped it for me. I've watched it so many times that 3 year old ds walked in on me watching it and said, "Did this movie come with the house?") Anyway, if you missed it on TV, look for the video when it comes out and be prepared for 6 hours of excellent entertainment.
The Lion in Winter--Peter O'Toole and Katherine Hepburn
Edited to say "Gaah! How could I forget 'Howard's End'"?