The out-and-out winner, absolutely no contest for me was The Testament of Youth by Vera Brittain. It's an autobiographical account of a young, middle class woman growing up just before the first world war and into the war. Basically ( and I don't think this is a spoiler, given what we all know about that war) by the end of the war, so many people she knows are dead - practically all her male contemporaries. I didn't just cry, I sobbed and had to go for a walk round the garden to recover. And what made it even worse of course was that it was all true.
I do quite often fill up when reading a book, but not often does it actually spill over into tears. The Book Thief (like many others here) was the last one to do that - even though you're told what is going to happen, it still is harrowing! Ooh I've just remembered another favourite one from my past - The Once and Future King by T H White - the story of King Arthur - I find it sad at the end when hopes are replaced by the sordidness of the world but there is just a glimmer of hope for the future - real tearjerker and a brilliant book!
And I tend to cry more at books than films - I think because you can slow down, and dwell on the description of what's happening rather being taken along at the pace of the film (does that make sense?). Though I do find that since having children, I am MUCH more prone to tears generally. Just don't show the cancer ad when the man is at the football match without his dad, and the bride is trying on her dress without her mum - Aargh I'm welling up just thinking about it. Bit close to home that one.