I would have to say that the scene between Ban and Elaine from "The Seven Deadly Sins" is the most romantic. After being fatally wounded by a demon she found the strength to crawl over to Ban and give him water from the Fountain of Youth to prevent him from dying; a deed that in turn would cause her death seeing as both of them could not both drink the water since it would only be half effective.
What really got to me from this scene was not only her sacrifice but his as well. He was willing to die in her place and gave her the water first before she saved him. And at this point they had only known each other a few days... She had lived in solitude for centuries and he had been the only contact she had made with in all that time. They fell in love because they both accepted each other for whom they where and I think that's truly beautiful.