Here's an amazing cover of Marceline's song "I'm Just Your Problem," arranged and recorded by Elena from Spain. I'm putting this up, not only because it's the most powerful and amazing cover of an already powerful and amazing song, but also because a bunch of people have posted Elena's version on their channels, and subsequently gotten more views than her. :) (and some of them weren't even crediting Elena at all, which is just plain dickish.)
The original song was written by regular Adventure Time composer Rebecca Sugar, and sung by Marceline's voice actress Olivia Olson. If you want to download this cover version for free, there's a link provided in the video description.