That skiddle diddle doo all song is burned into my head. I actually have it but I can't NOT think of it! Help!!

On topic, this game has some incredible music at times. Love it.

Truly one of the most beautiful songs

I don't know the names, but the caves' theme, the battle theme that says "I need a bigger gun" very catchy. Also Noctilum is great though I think I heard another version in one youtube video that I liked even more.
This soundtrack is pretty good though I miss the masterpiece that the previous game was. Every song was memorable and epic.

Too hard to pick for me so many awesome songs, I may post the ones I like the most later

Yeah, I love the Noctilum daytime theme. Oblivia's daytime theme is pretty awesome also.

Despite not playing it, I've listened through the whole OST :p

Definitely good, though halfway through, a lot of the songs felt the same.. so my pick probably goes to

The Key We've Lost
or
PianoX1

probably the latter

Sylvalum's night theme song. It's OUTSTANDING.

Almost all of them. I love every battle theme based on enemy type and your current gear.

This is gonna be hard...

From what I heard in the game so far:
1. Sylvalum Night Theme
2. ThemeX
3. Noctilum Day & Night Theme
4. Black Tar (Skell battle)
5. Wir Fliegen (Overdrive theme)

Kind of odd no one has said the Promordia theme (both day and night), I think it's spectacular. It has that epic sound to it..

Not sure if it's my favorite but it's up there. The Key We've Lost is also fantastic. Overall I really think this music is the one of the premier soundtracks in gaming this year. Absolutely stunning!