A simple question and topic that I think will help many people find a lot of awesome songs to listen to. What is your favorite OST in an anime? My favorite is Fairy Tail by a long mile. It is well known for its amazing music, and it never disappoints me when it comes to amazing music. Some honorable mentions for me are Death Note and Blue Exorcist.

Probably Initial D. It's mostly just a bunch of eurobeat stuff but it's pretty great

From what I know of so far, I'd definitely have to go with Death Note. 

The ominous melodies, and songs like Light's Theme or L's Theme just make the sound track amazing to listen to. I've probably listened to the first CD of the OST about 30 + times all the way through. It's great to have playing in the back ground while doing things like gaming or work. Speaking of, I'm probably going to listen to it again here soon since I'm thinking about it lol. 

Aw man...thats kinda hard... I have way, way too many. There is Sailor Moon, then there is Fairy Tail, then there are others such as Magical Girl Lyrical Nanoha, Blue Exorcist, Precure etc. @.@

Wow, I love so many OSTs that it's not even funny. Off the top of my head and in no particular order for some of the songs...

Anything by Joe Hisaishi
Nodame Cantabile
Ghost in the Shell
Inuyasha (Kikyou's Theme)
Cowboy Bebop
Samurai Champloo
Angel Beats!
Accel World (Blood History, Rainbow Aurora, more like Hiroyuki Sawano's music =P )
Attack on Titan
Death Note (L's Theme)
Guilty Crown (krone and Bios)
Gensomaden Saiyuki (Shizuka Ni, Hakkai's Theme, Hakkai Flashback)
Death Parade (Flyers, Moonlit Night)
Naruto (Sadness and Sorrow)
Tsubasa Chronicles (You Are My Love)
Fullmetal Alchemist (Beaming Sunlight, Brothers, The Alchemic World)
Sailor Moon (Moonlight Densetsu Music Box Version)
Black Cat (Kuroki Neko)
Samurai X Trust and Betrayal / Rurouni Kenshin (Sound of Snow Falling, Kotowari, Quiet Life, and actually, pretty much every song)

And so many more... I guess I'll add more once I think of them.