Games with great OSTs?

 Diabolik Lovers and Starry Sky in Spring    hehe Hikaru-San voice was great there  

Until now? I'd say Nameless and Starry Sky 
If mobile games count then I'll add In your arms tonight from Voltage <3

Whoop whoop whooop!

One of the best OST's I've listened to, and still actively do, is Lamento's 'The World Devoid of Emotion'
Even if your not a BL/Yaoi fan the music for that visual novel is amazing. Same with DRAMAtical Murders >.> -cough-

I've bee listening to Black Wolves Saga's OST a lot too, I really enjoy songs with vocal, even if it isn't singing words.

For this time, NORN9 is a game with the best music (bgm, opening&ending song)
The second is Glass Heart Princess... And the third is D.C. ~Da Capo~ Girl's Symphony (alright, the character ending song really good).

I am absolutely in love with Ozmafia's OST. One of the most memorable out of all the visual novels I've played so far. I also like fortissimo EXA//Akkord: Bsusvier's soundtrack, but it's a galge.

I need to agree with kyuuichii. Norn9 has a SUPERB soundtrack,   and really, you can just listen to it without ever playing the game, the music is awesome in itself 

Right now I'm absolutely loving Ayakashi Gohan's OST ^^ It's really soft and peaceful. Sometimes I stop playing just to listen to the soundtrack and attempt to play it on piano for a little while.

Alice no kuni series.... they have wonderful music! Hakuouki does too.... especially the character themes. The game music makes me want to keep playing into the wee hours of the morning.

I can't think of a soundtrack as a whole, but there are songs that just make me happy. The theme song to Dandelion 'Yet, I Can't Stay' is awesome, and I think at the end of Idol Days Sim Date SquaDus's cover of 'Rolling Girl' plays. I single those two songs out as I listen to them outside of playing the games. 
The cover of 'Rolling Girl' is one of my favorite songs in general I think.

To this day, the best Otome OST I've ever heard is The Second Reproduction. I loved that music SO much. I set some of it as ringtones at one point or another.

^I second The Second Reproduction hehe...