This remix is absolutely wonderful, and one of my very favorites on the site. A neighbor in my apartment building heard me playing this once (I had it up pretty loud), and actually knocked on my door to ask me what is was, and remarked at how beautiful it was. Damn fine job.
The three bosses at the end of Ninja Gaiden for NES. That was the toughest time I've ever had beating a game. Not to mention that you had beat all three on one life, and if you died at any of them, you started way back in the middle of that hard-as-hell final stage. That was back when games kicked your ass and you liked it.