When I asked this question, I was going to reply, saying Jurassic Park, but that same night, the film showed on TV, so I got to see it in the end. Other films I haven't seen that fall under this category include:
Psycho (seen bits and pieces, but not the whole film)
Planet of the Apes
All the Star Wars films
The Great Escape
Terminator
Alien
Like dgrwriting, I also haven't seen Gone With The Wind, but I adore The Wizard of Oz.
With the exception of Terminator and Alien (which I would watch if they were on at a convenient time on TV), I am not interested in the seeing the others.
Oh! Bryony, the original 'Planet of the Apes' was voted the film with the best-ever ending! See it, if you can. It's excellent. Charlton Heston's 'gruff' voice in one scene was not 'put on'. He had a sore throat! Can't say anymore, or it'll spoil it for you! Cheers!