Firstly, for me nothing beats Pandora Hearts. The songs are composed by Yuki Kajiura and that woman is a genius. Everything she does sounds amazing.
There is one song that haunted me for a while. That's the song from the scene where Cheshire is left to die. You can hear the cat in background of the song and it gives you goosebumps. Jack's song is amazing too. The whole soundtrack is just so epic.
Another anime that caught my attentiom with the soundtrack is Amnesia. The anime has a good plot but I got annoyed with the Heroine. She was too clueless about everything and I think she didn't deserve any of them. And Ukiyo is such a great hero in my opinion. He dies in every dimension and has an alter ego yet he fights for her and saves her every time. I'm really curious how was reality.
Anyway, the soundtrack is very good. Some songs are so well made that make you want to place them in some other epic movie. The story is mindfuck and so is the soundtrack. But I do believe that the music saved the anime. Of course, the guys did too but for me was definitely the music.
Now, one anime that I just can't seem to finish is Dantalian no Shouka. The soundtrack is magical. It gets you transcending into the actual story. I read a lot about the background of it and it happens. The information throughout the anime about Dalian and other things regarding her are actually fonded on something true. It made me love it even more.
An anime that changed my life is Attack on Titan. When I first watched it, I laughed. The opening was so weird. But, I just found myself searching for more. The soundtrack is pretty epic. The anime and manga are pretty epic. I just can't wait for the movie.
Death Note is my favorite anime. I think everyone who met me knows how obsessed I am with Death Note in general. The music doesn't disappoint either. L's theme was one of the few songs I learned to play on piano when I didn't even know how to properly play piano. This anime means a lot for me and of course, the soundtrack is unforgetable.
Remember Ao no Exorcist? I know everyone loved it. I did too. I was at day with the episodes and I was always left wanting for more. I liked the anime because it had a really good opening. It just popped in front of you and you wouldn't be able to scroll down. Let's be serious, it's an amazing anime and the plot is very good. The soundtrack is pretty much epicness. The story needs it. I think that anime goes amazingly well with the music. I mean, it was well chosen.
I can't ignore Vampire Knight either. That anime really got to me back then. The whole story was amazing. The manga is on going and it fucks with my feelings. I swear I'm going to enter my screen. However, the soundtrack is noble and it would work in real society parties too. It has that melancholic and misterious sound and I just love it. Everyone does because it gets you into an inspirational mood.
Maybe it's not really one of the best but Gravitation was my first anime. I kind of started with yaoi lol But let's be honest. The songs from that anime were pretty good. I got to the point where I actually knew the lyrics. It was a big deal for me. And Shuichi's theme is pretty awesome.
Now I know I may not place them all here. There are more anime with good OST. I can't go through all of them right now but I will at some point. These are only my absolute favorites.
As a side note, Naruto doesn't have a lot of extraordinary songs and neither does Kuroshitsuji. They are good but not to the extent where I would listen to them repeatedly. The anime are good and I love both of them and even more, but I don't have goosebumps from them.
I'd really like to know other opinions over OST matters.