Rules:

Only 2 per post. Don't want to cluster fuck it.

THEMES only, so not soundtracks. (So for example, for Final Fantasy, only the main theme, not Aeris's)

Provide an explanation. At least a sentence.

Don't need to play the game. By just hearing the theme, you know what the game is about.

It's over the top, in sex, gore, music, theme, scale, etc.

TBH, there wasn't exactly a main theme, since Chaos Theory had a bunch of theme for the main screen. I'm just going to go with this though, since that's what I remembered, I can only post 2, and that's what I got on top for Youtube.

Seeing Fisher's old face and this song, just captures everything about the game IMHO. It's dark, it's gritty, it's not nice, it's not smooth. We see an old veteran, a relic of the Cold War, in the middle of a sci-fi cyber war where the nature of war has only continued to become different.

This is incomparisson to fucking this:

Such fucking emo shit. I'm not against melodrama, in fact, I fucking LOVE melodrama. It's just that it completely defeated everything the past three games were working uptowards.

You already posted GOW 2 so here is my other 2 main themes:

Also one of the best tracks in a video game

Whenever I hear it I just want to drop everything and join the army. DICE better bring it back in BFBC2 because I didn't hear it in the beta.

I don't think you have to worry, they had it for BC1

I prefer Orchaestra versions of themes so I'll post a couple of those.  Really there are too many great video games to really pick favorites, so I'll just list two that are really easy to find on youtube.

Secret of Mana

Fire Emblem 6

It's the Super Metroid theme, but it's the Metroid theme overall so I believe it fits the description.

What can I say? One of the best themes from one of the best gaming franchise of all time.

Zelda was already posted. I guess Halo will be posted soon so I didn't bother finding a video. Also, honorable mentions: Bionic Commando, Final Fantasy X's "To Zanarkand" (that's the main theme methinks. If anyone posts it, put the orchestra version please!), Hell March (Red Alert), Super Mario Bros..

I have to post a Youtube vid?

music to my ears!!!!!!!!!!!!!!

Above: the original GTA, back when it hadn't yet even seen a Playstation release. Gotta love a crazy track like this to pump you up while being chased by the cops and slaughtering innocents!

I don't know if this counts as its main theme, but you hear it the most often as it's the Town music, so everyone remembers it best. Well, it's also remembered because it's fucking awesome, of course. It complements the atmosphere of Diablo too well...

<--- most EPIC intro ever, the opera in the beginning is so beautiful

Holy shit I remember that song (especially before the vocal) GTA still had an addictive gameplay long before GTA III. First time getting a tank or those 7 or 8 guys walking in a straight line behind one other, the game was fantastic.