Since not many people answered, I'll just answer my own question with my own favorites lol.
One of my favorite Nôm characters is đĩ , which is a vulgar term for “prostitute.” Why is it so interesting? See for yourself:
It's a 女 character with a dot inside. How does this represent “prostitute?” Well, 女 means woman, and when a woman has her “dot” exposed, it's likely she's a… well, you understand. The character is so creative in how it's created, in my opinion.
Another one that I like is the Nôm for mẹ , meaning “mother.”
It has 女 and 美 put together. It's obvious that the reason for its creation is 女 is for semantic, referring to a woman, and 美 is for phonetic, referring to the sound mẹ (which sounds like Vietnamese reading of 美 mĩ). However, 美 has another meaning in that it means “beautiful,” so it's like saying one's mother is the most beautiful woman in the world.