Castlevania II: Simon's Quest
I could easily write 5,000 words here about Simon's Quest, which is one of the most interesting of all video games. It was a fascinating game because it was a different game; there has never really been another game quite like it. Zelda 2 was sort of similar, except Simon's Quest had no overworld map or anything of the sort, and also the NPC's in Zelda 2 would, once in a while, tell you something useful. Whereas almost every single NPC in Simon's Quest--and there are quite a few of them, for a mid-era NES game--is a godless communist liar.
Anyway. I think it's interesting that the most iconic piece of music from the ghastly long Castlevania series originated in its most obscure game. Everyone that's ever played a Castlevania game, or heard of Castlevania games for that matter, knows Bloody Tears and has probably heard several different versions of it. Surprisingly few of the casual Castlevania fans know that Bloody Tears traces its lineage all the way back to Simon's Quest.
Bloody Tears might be the most oft-remixed piece of music in gaming history; there's a different version of it in almost every Castlevania game, to say nothing of the myriad fan remixes. And it deserves its lofty status in the gaming world: it is, hands down, the finest piece of 8-bit music ever composed. One of the many interesting things about Simon's Quest was that the music was genuinely blow-me-away for its time.
And if you're wondering if I finangled the list a little bit to put this piece in slot #13, to line it up with the 13 clues to Dracula's riddle*: Yes. Yes, I did.
* Half the NPC's in the game tell you about Dracula's riddle, and you can even find "clues" to it here and there, but the joke is on you: there is no riddle.