The Butterfly Effect
This is the darkest movie that I casually rewatch.
This movie is heartbreaking because at its core, it's just a guy wanting to live the perfect life with the girl he loves, and he just can't get it right. Which makes the ending (director's cut, which is the one you should watch) even more tear-jerking. Then the score (especially that one that gets played a couple of times throughout the movie) is excellent. Ashton Kutcher, in one of his first dramatic roles, knocks it out of the park for me, along with the rest of the cast. I'm glad this has been reevaluated positively (at least on here) for being a good film.