Kingdom Hearts: Birth by Sleep
This transcription started as a bit of a joke, but I'm actually quite happy with the result! I give you the theme song from one of my favourite games, 'Overcooked', played by one person with the right hand on the piano and the left hand on the melodica. It may look a bit stupid, but I was surprised how normal and like a harmonica it sounds (in my humble opinion).
Plants vs. Zombies
Super Mario Bros
I'm not a gamer myself, but this game music blog contains lots of sheet music!