Ladies and gentlemen, I present the greatest track in video game history whose title includes an exclamation point.
EarthBound does an exceptional job of priming you for the ultimate confrontation with Giygas. Most RPG's I can think of don't make clear who the Big Bad is until you're at least a little ways in; EarthBound throws him at you in Chapter One. Whereas Lavos is a distant disaster you're trying to prevent, Kefka is an evil god you're trying to reach high up in his tower to dethrone, and it's been almost 15 years and we still don't really know what the hell Sephiroth's deal was... Giygas is an ever-present malevolent force throughout EarthBound. The game reminds you of this in several unusual ways, notable among them in straight text after you drink some drugged coffee or tea (no, really). "Your enemy is also struggling." "You are drawing near to Giygas."
And when you finally reach Giygas and are confronted by his twisted henchman Pokey (or perhaps more properly, Porky)... well, there are several video games in which you fight an alien. Your encounter with Giygas is by far the most alien of them all. There's no contest. It's eerie, it's upsetting, it is frankly terrifying.
And enter this music.
At first you think, especially if you played Mother, "huh. 8-bit music in the final battle." And indeed it's very good 8-bit music with a certain haunting quality to it. Then you get about 50 seconds in, and it kicks your ass. Alien thrash metal, ladies and gentlemen, and if you listen carefully you'll notice it's set to a similar beat to many other tracks in the game. Good soundtracks are like that.
This is only the first of four stages of your fight with Giygas, the all-mighty idiot, whose own mind was destroyed by his incredible power. The background noise gets creepier in every phase, but it can't really be counted here since it's not really music.
The endgame of EarthBound was magnificently done and remains unsurpassed.