The music that plays whenever Kenshiro is about to kick ass! SENTOO! (which is sometimes mistaken as Ai wo Torimodose Instrumental, which it essentially is). FATALK.O.
The moment Kenshiro reveals Muso Tensei for the first time against Raoh: Kareta Daichi - Parched Land. The song fully embodies how much sadness Kenshiro has born upon himself, and turned into enough strength to send Raoh SHAKING in his boots with SHEER TERROR. Also a crowning moment for Kenshiro's voice actor Akira Kamiya singing this in a very soft yet determined voice. He pretty much speaks what Kenshiro has in his heart at this moment.
That song was created and played by Tak Matsumoto, the guitarist of the most successful band in Japan, B'z. The guy has been considered by many to be a genius guitarist and the song certainly showed it. B'z itself would later sing the theme song to the 2007 Raoh Den movie, LONELY STARS, which is very awesome as well. Here's the live version 
The French opening can't compete with "YOU WA SHOCK", but it's still upbeat, and manly as well. And this is an amazing achievement when we make a comparison with all the anime from that time that got a new, and in most cases craptastic, opening.
They may not be original to the series, but Gackt's Lu:Na and Oasis, the OP and ED from Shin Hokuto no Ken, certainly qualify. Adding into this is Kenshiro no Theme II, a theme that's ideal for every fight in this OVA.
The pachinko game by Sammy Corporation features a pair of songs from Japanese-French band Drive Far as its themes. Ladies and gentlemen, Thrust of God and Blazing Circle, the lyrics of which were both written by Tetsuo Hara himself.
Minagoroshi No, the theme that plays when Toki clears the road for Kenshiro, laments Toki's lost potential as the original successor due to his radiation sickness, whilst at the same time showing off his extremely graceful fighting style.
Purple Eyes, the song that plays over the credits of the 1986 movie, is both beautiful and heartbreaking.
Hokuto. Ansatsuken. Even in a soundtrack with legendarily hot-blooded music, the fast-paced strings on this one make any scene where it plays all the more badass.
The OST from the unfinished English dub, while nowhere near as iconic as the original, is filled with amazing drum and bass tracks. Special mention goes to Urban Utopia, which contrasts quiet suspense from the synth strings and horns with the urgency of the drum and keyboard parts, and wouldnt be out of place in Street Fighter III.