Hmm... I can't pick just one! (Sorryyy)
I have so many that I love. Music is my life!!
One big big favorite is "Gravenimage" with Sonata Arctica, a finnish (power-) metal group~ It's so beautiful. The lead singer writes all their music and lyrics, and the composition as well as the lyrics are really really beautiful! ^_^
If I were to chose a favorite Korean song (the two biggest genres I listen to is rock/metal and korean pop)... it would be so difficult... but.... hmm.... Generally, my all-around favorite korean pop GROUP is a group called SHINee, but my favorite -song-, singular, would probably have to be SS501's "Love Ya", because its composition is a work of art.
The classical music blends perfectly with the pop beats, and they switch from the deeper singing voice of one member to the lighter coarser voice of another in a really brilliant way. Though to notice all these details, one really needs to listen to music in a sertain way ^^;;;