this is about to be the longest post I've ever done LOL
Dynasty Warriors has some good stuff, especially if you like guitars.
From DW4, Struggle for Existence is my favorite.
http://www.youtube.com/watch?v=DS8qIQK_aM0
The soundtracks are pretty varied through all the Warriors games. The ones I would recommend are Dynasty Warriors 3 and 4, and Samurai Warriors 1 and 2. DW5 and 6 have some good tunes as well but... I find the majority of their tracks to be underwhelming.
As for Samurai, my favorite track is Komaki-Nagakute
http://www.youtube.com/watch?v=25VYtwXsuZ4
There are two versions of this song. The remake (that one) and the original. Original is a lot harder to find but I do have it if you wanted. They're both good.
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Mega Man. Particularly games 2 and 3. These are must-haves. You should already know these but just in case, I'll post a little something.
Sparkman (MM3) :
http://www.youtube.com/watch?v=qUha-uphaSg
Now it's impossible for me to mention Mega Man music without mentioning my 2nd favorite band of all time : The Megas. They do Mega Man songs and add lyrics to them. And it's ****ing incredible.
Here is the original Quickman song from Mega Man 2.
http://www.youtube.com/watch?v=U-uZF2rE7dQ
Now here is "The Quick and the Blue" by The Megas. See for yourself.
http://www.youtube.com/watch?v=lKkKegPx7p4 (quality sucks but whatever -_-)
I mean seriously. "Your speed means nothing if it's DEATH that you're running from!"
They've done a Mega Man 2 album, a split joint for Mega Man 1 with the band Entertainment System doing the instrumental part, an Acoustic version of their Mega Man 2 album, and they just released the Sparkman single for Mega Man 3.
Then there's Mega Man X series which is also wonderful. 1-6 all have great songs.
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Katamari Damacy soundtracks are usually pretty fun, especially the first two games.
http://www.youtube.com/watch?v=7_QydNXI_ok
Guilty Gear, in case you hadn't heard, is ****.
http://www.youtube.com/watch?v=BVe78_xv1Lg
Metal Gear Solid has a lot of generic "oh crap, I'm a spyyyy" music but it also some absolutely breath-taking songs. Of particular note are the Metal Gear Solid 2 and 3 opening songs. If you doubt me, wait for the ending.
http://www.youtube.com/watch?v=rNVCD1dsnJI
flat out one of my favorite songs of all time.
Actraiser has some nice underrated music. Fillmore is awesome. And this :
http://www.youtube.com/watch?v=bF9UUz5eHQg
Zelda : Wind Waker has one of my favorite songs of all time.
http://www.youtube.com/watch?v=-6C4fRiLF_g
It's just... so tragically beautiful. I can't even... lol
Zelda, Mario, Sonic, and Final Fantasy music is often beast but I am going to pray you know that already. Chrono Trigger deserves a special mention for having a ludicrously awesome soundtrack. Metroid, Super Metroid, Metroid Prime... Donkey Kong Country 1 and 2. Pokemon Heart Gold / Soul Silver have a pretty nice OST too.
And of course there is OCRemix. You better know who they are. If not, well... ****ing go there lol. Tons of wonderful music for pretty much anything. Especially if you're fond of Chrono Trigger, Castlevania, Mega Man, Zelda, etc you can find some really amazing remixes.
also of note is the band Powerglove which does metal covers (no lyrics) of old game songs. my personal favorite is Power, Wisdom, Courage, which is their Zelda medley. Pretty ******.
http://www.youtube.com/watch?v=6doJ0ErfSHg
might post more later if I think of something