I'm surprised no one has mentioned Donkey Kong Country: Tropical Freeze. I don't know if I'd call it the greatest soundtrack ever, but it's definitely one of the best in the past few years or so.

Earthbound

Any answer will obviously always be extremely subjective. No one has properly listened to every single gaming soundtrack in history.

For me it's very easy:

Metroid: Prime

Bayonetta or Xenoblade Chronicles.

I'm going to take the easy way out and say: pick a Star Wars game. Almost any Star Wars game.

How many video games can claim to be scored by John Williams?

There's so many good ones. However, two big ones come to mind:

Nier
Final Fantasy XI (if you include all the expansions)

Xenoblade, Chrono Trigger, Tales of Symphonia, Chrono Cross, Breath of Fire 3, The World Ends With You, Xenogears, Super Mario Galaxy, Ys Seven, Trails in the Sky, Final Fantasy 9, Castlevania Symphony of the Night, F-Zero GX, Zelda series (Pick any), Earthbound, Final Fantasy 6, Ys three, Streets of Rage series, Final Fight series, No More Heroes, Pokémon X/Y, Super Mario RPG, Paper Mario games, Dk Series (Fav is DKC2), Kirby series (my fav being Return to Dreamland), and the most criminally underrated one of all: Arc Rise Fantasia.

My favorite personally is Ys Seven with Arc Rise Fantasia and Xenoblade a close second.

Hmm difficult one.

Tekken 2
Duex Ex: Human revolution
Tenchu PS1 (or was it PS2, the first tenchu tho)
Vagrant Story
Metal gear solid theme song (i doubt any human being can listen to this and not like it)
Zone of the enders: 2nd runner

Kingdom hearts

Such a hard question... I'd have to think about that for a while lol. The first 2 titles that come to mind though, are Metal Gear Solid and Super Mario Galaxy.

Ok if we were talking best original scores or pieces, the game selection would be massive, but in terms of complete soundtracks, there's really only a few that hold the title.
Echo the dolphin
persona 4
Metroid prime
earthworm Jim
jet grind radio
journey
katamari damacy
Catherine
symphony of the night
mega man x2
Ff10