Code Geass was really awesome and unpredictable, until the last two or three episodes and it felt way too obvious that they were building up to it.Rolo's death scene though, that was fantastic.
The Dark Knight was a good movie overall and the boat
scene was IMO the best/most memorable part by far, though I thought it was towards the end of the second act, but I haven't seen it in a long time and I have trouble motivating myself to watch it again.
My personal least favorite ending off the top of my head is Return of the King the movie, but at the time (and even now) I generally don't watch movies over 3 hours in one sitting so that ending dragged.
Grave of the Fireflies demands your attention.And also Now and Then Here and There.
Final Fantasy X (Great ending) - Who ever has played it, which I am sure most of us on here has everyone knows why. Its a little sad what happens to Tidus but overall great ending.
Man.... so good. No ending of anything has ever emotionally destroyed me like that.