Ninja Gaiden/Awesome Music

Ryu's Determination (Act 4-2's BGM) from the first NES game.
- The version used in Ninja Gaiden Shadow for Game Boy, Stage 3.
  - Arranged version of Stage 3.
  - And leave it to Koei to remix Hayabusa's theme into more awesome in Musou Orochi 2 here
Reflection (Act 5-3's BGM) from the first NES game.
Thunderstorm (Act 2-1's BGM) from the second NES game.
Act 4-2's BGM from the third NES game.
Chaosium Sword (Intro Theme) from the second NES game, Dark Sword of Chaos.
The rather sad tune Tragic Fate (Act 6-4's BGM) in the first NES game.
All three games have a great boss theme.
The Parasprinter (Act 3-2's BGM) from the second NES game.

The three games' Main Themes of course.
Alma Awakened has an appropriate enough atmosphere for said boss.
Emperor, another of the first game's iconic themes.
Pray to God has a melancholic tone and a very appropriate title, considering that you hear it just before fighting Alma…
Arachnoid and A Masked Curse are pretty epic for first-level boss themes.