Anime OST huh, it's quite hard for me to say.
I usually only stick with Opening and Ending, not the whole set of the soundtrack.
If that's the case, I would say that the Anime Bakemonogatari has a consistently good OP and ED. They got the recurring artist like Claris and Trysail. Besides that, because most of the OP and ED are sung by the voice actress themselves, it felt more touching. Because you associate the songs with their character, it just makes it sounds more beautiful.