My favorite out of all the song's I've heard has to be SHINee's Romeo + Juliette. It's just such a beautiful song from start to end. Really, SHINee makes some of my favorite music in all of K-pop, their discography is really good. Before I decided Romeo + Juliette was my favorite K-pop song ever, my favorite was another SHINee song called Close the Door. I really love that one's arrangement.
20cm - TXT
LA DI DA - EVERGLOW
Psycho - Red Velvet
Pink Blusher - IZ*ONE
There’s honestly a lot more than these 4, most of them are from IZ*ONE, Red Velvet, LOONA and GFRIEND.