BTS - Butterfly
Perhaps the hottest boy group right now is BTS. And while they're often known for their insane choreography and in-your-face energy, they have some really great stuff on the slower side of the spectrum. "Butterfly" is a classic example and both the singers and rappers restrain themselves wonderfully. Despite the mass amount of falsetto, it's a rare instance where it works given the pace of the song. BTS shows that you don't need excellent vocalists to have excellent ballads.
Younha - Realize After Breakup (feat. Kim Bumsoo)
As mentioned previously, Younha has seen her fair share of strong collaborations. This was a top-end ballad of 2009, and the names alone reveal why. Contrasting Kim Bumsoo's sharp voice is Younha's more clear and rounded one. They combine for wonderful harmonies throughout the song and easily make this piece a karaoke classic. Neither singer overpowers the other, which is a quality I really love seeing in duets (even if it's rather uncommon). Impressive singing from start to finish, what more could you ask for?