It's out of Super Metroid and Metroid Prime for me.

Metroid Prime takes the cake.
Most of the Music for the bosses is awesome, and the Space Pirates Theme, Phendrana Drifts, and best of all, The Credits Theme. That song that played during the credits made me actually enjoy watching it. That song is so amazing, oh my goodness, I would throw a fit if I couldn't listen to it. I mean a big fit.

I'm glad I'm not the only one that considers this theme to be the best out there. Too bad they didn't really include it in Metroid Prime 3.

I am currently listening to it now my friend, here is the link: http://www.youtube.com/watch?v=6VQWGGKABqc (Look out for my comments featured on the comments page).

Metroid Prime hands down.

I almost put the title theme to prime in the OP instead of phendrana drifts, but I just thought the drifts was the fan favorite song from Prime 1. I do greatly prefer the title theme though, freakin' epic. Makes me want to go play it right now.

Metroid Prime and this is why:
http://www.youtube.com/user/SupraDarky#p/u/926/P8oefrmJrWk

I would consider this as the most tranquil piece of music in a Metroid game: http://www.youtube.com/watch?v=LudFV61zsaw&feature=related

Super Metroid.

But I must say that the Metroid Other M trailers have the best soundtracks. For example, the re-make Mother Brain final battle at the beggining; its just perfect.

And the music for every trailer fits perfectly.

who's doing the soundtrack for other M? I must look that up. I really hope other M has a mind blowing soundtrack. That will make it or break it in my mind.