This topic is interesting as I have many favorite songs, but mostly are either from ending or opening theme. I don't know if you can consider that (opening/ending) as part of OST's as they normally come in separate package of the CD...but if you do, I think I might end up spamming this thread with many posts....LOL
Back to OST, I found that many recent dramas don't really have something that can catch your attention as compared to older dramas...not saying it wasn't good but it's just that...well.... unnoticeable...While they do have good opening/ending songs, the BGM is nothing to compared to 90's and early 2000's dramas...with the exception of Taiga dramas of course...
That being said, here is one that I like as a starter....from Dr. Koto Shinryoujo