Most OSTs are extremely formulaic, but most of my favorites are those that go against the usual sappy ballad or "uplifting" ending uptempo. Not that they're ground-breaking songs in themselves, but that they show that much more creativity than the usual "crying in the rain" track and prove that no, not all OSTs are boring.
Kyuhyun (Super Junior) and Tiffany (Girls' Generation) - "To The Beautiful You"
From To The Beautiful You
I've probably said this before already, but really, To The Beautiful You may have been underwhelming as an drama but it had one of the best drama OSTs I've ever heard. And it wasn't just one track that stood out -- apart from the title track I also really liked Jessica and Krystal's "Butterfly" and J-Min's "Stand Up." It was a well-rounded, extremely well-produced but also refined and musical OST. They weren't just tracks meant to play in the background, they were tracks that helped carry the drama. "To The Beautiful You" in particular is probably my favorite because it oozes style. It reminds me a lot of theme songs for older American TV shows -- I can imagine the opening credits of Brady Bunch or something whenever I hear the track. "To The Beautiful You" is catchy, it's easy to sing along to. It's not overpowering as a song though, whether in terms of melody or arrangement -- it's subtle but effective. This is the type of song you can listen to any time, in practically any situation and it won't be a bother but it won't disappear into the background either. "To The Beautiful You" is a balanced song, in every way.
Kim Jaejoong - "I'll Protect You"
From Protect The Boss
The thing with drama OSTs is that when you watch the drama they usually only play one specific part of the song either because it's the only part that fits the drama or because it's so flat that the entire song sounds exactly the same. But what I like about "I'll Protect You" is that, probably also because I watched Protect the Boss as well, is that they used various parts of the song -- because they could. Over the course of the drama I heard the verse, the chorus in all its varying intensities, and that's telling of how "I'll Protect You" differs from the stereotypical OST. Not so much in terms of sound, it's the usual ballad with the usual line-up of instruments. Maybe a bit more emphasis not the drum line, but the mellow guitar line and strings are also in full-force. "I'll Protect You" is different because of its structure, how every part can be used for a variety of different scenarios. I also like this track because, well, it's Jaejoong doing what he does the best -- being his versatile self. He goes from quiet, almost whispering, crooning to belting that high note at the end of the middle 8 with so much clarity. Jaejoong helps turn "I'll Protect You" into the dynamic song that it is.
DBSK - "Athena"
I was expecting a lot from "Athena" because it was DBSK's first release post-split, as just Yunho and Changmin. My expectations went down when I heard it was a drama OST, but whatever the case it definitely exceeded each and every one of my expectations and eliminated whatever doubts I had about Yunho and Changmin's capabilities. Sure there's nothing like the magic of five-piece DBSK, but "Athena" was the track that reminded me, reminded everyone, that Yunho and Changmin were part of that five too and could very well carry themselves. But one of the reasons "Athena" became a defining track for DBSK (though a bit under appreciated, I think) is because it was probably the furthest from the stereotypical idea of an OST. It's fitting that this is for a thriller/crime-type drama though -- the strings running around all over the place, the second layer of dramatic strings, the stomping bass and melodramatic piano are regular fixtures in action scores. But to see that translated into a pop track, for a K-Drama, isn't something you hear very often from idols who do OSTs. And I think also that had five-piece DBSK done this it wouldn't be as sharp and hard-hitting, it wouldn't have been as effective, as it was because of the nature of Changmin's lead combined with Yunho's foundations (and even his higher range, actually). "Athena" was the song that set the direction for 2-piece DBSK, and I think it did it outstandingly.
Ailee - "Ice Flower"
Technically, "Ice Flower" is a generic drama ballad but it's prevented from becoming sappy because of Ailee's execution. Ailee turns "Ice Flower" into a diva song because her vocals are more powerful, more dynamic and more ear-catching -- it becomes a lot more complex than just straight-out crying precisely because of the mastery of the execution. Add that to a gripping melody that's also more eventful and dynamic on its own, and "Ice Flower" is probably one of the best examples of an OST that breaks the mould by doing it better than everyone else.
IU - "Someday"
From Dream High
Like To The Beautiful You, it was hard to choose just one song from Dream High -- after all, a drama about idols and a music school is bound to have more variety and musicality to its soundtrack. But IU's "Someday" was the track that has remained a favorite of mine, regardless of whether as an OST or as a Kpop release. "Someday" is one of the earlier tracks from the IU that I enjoy, the IU before she got pretentious. It's a beautifully-sung, well-produced track with just the right balance of maturity and vulnerability to the execution. IU's vocals sound her age -- not squeaky young like a lot of her earlier releases, but she's not trying too hard to sound old either. The song itself is quaint, and the "acoustic" aura gives a fresher, younger twist to the package. Like "To The Beautiful You" it's an easy listen but not one you'll forget anytime soon.