So the love story I am currently most digging is actually the Ice King's background story, which has only been explored a few times. When you first meet the Ice King in Adventure Time, he's a hilarious weirdo that is always kidnapping Princess to try and force them to marry and love him.
As you learn his history, he was driven mad by the magic in the Crown (which give him his powers), turning him into the Ice King. He may have saved the world in the past as well. He might have stopped the Lich King as well.
As he lost his mind, he also lost the love of his life, Betty. No one is sure what became of Betty.
Now, almost a thousand years later. Ice King is a lonely, kooky magic-user, with domination over the power of ice and snow, living with penguins in his Mountain Fortress in the Kingdom of Ice.
But every now and then, he's haunted by fragments of his past, in particular, Betty.
I love it, because the Ice King is far more than he seems.