There are a lot of duets by different TVB artists for either theme or sub-theme songs, and for the most part, a lot of them are pretty good. There is one that is my favorite of all time, but that's not to say it's because it's by the best singers, best lyrics, or from the best series. However, this song is very good to me because the 2 voices used in this mesh very well in this song. That theme song is by Myolie Wu and Ron Ng from Lost in The Chamber of Love.
It might seem like a weird choice but one of the reasons I love this song is because of how they arranged their voices and how the song is layered. I like how they interchange lyrics, blend well towards the chorus, and it builds at the end with them both singing a different set of lyrics at the end. To me, it just sounds very beautiful when their voices are but together and of all the younger fadans and siu sungs, I think that this is the best match. I hope for their next series together (His True Father), they get to sing the theme song together!