For #KpopMonday theme #ThenAndNow I'm going to go with my ult, who hasn't officially debuted as a soloist, just in groups, but has released solo songs:
Ten - New Heroes (technically Dream in a Dream was first, but that was more performance/dance and New Heroes is a full song)
https://youtu.be/VM-g_bkFdzo
Ten - Birthday
https://youtu.be/QGySpyt0Byg