Who would be the best/worst OC to fall into the Underground?
(Guessing you mean the Undertale verse with this.
Well the worst to fall would have to be Gran. He’d likely kill any monster there to try and further his soul research then leave.
Best would be… surprisingly Scy. While Cataberry could have no problem fitting into the monster world she is still second place in this contest. Scy would be thrilled to be there. She loves anything not human plus her killing would stop completely. She would likely even give up her soul if asked just for the chance to have monsters ruin humanity’s day. However, Syph would likely follow her down if Scy suddenly disappeared from the headlines of human’s newspapers.)