Aside from the main title music, no song is more featured in Game of Thrones than The Rains of Castamere. With lyrics lifted directly from the book, it tells the story of a victory by Tywin Lannister over a rebellious House, and was memorably used as as a foreboding hint of foreshadowing in the tragic Red Wedding episode. You can read a detailed rundown of each episode it is featured in here, and brush up on the song’s full backstory as well.
There are many versions of the song floating around on YouTube, but my favorite is Tina Gou’s haunting cello version. Her arrangement of the music includes traditional Chinese instruments and her video for the song features an incredibly elaborate headdress.
I also love the compilation user Mostafa Adel created of several scenes featuring the song. It’s well edited, and manages to show the importance the song has played over several seasons of the show while also being spoiler free.