I do not know if this has actually occurred on screen or not (or if anyone already said this), but I think it would be interesting if the Doctor/EMH met Zimmerman (his creator). Zimmerman's ego would make him think of himself as the Doctor's creator/god, and the Doctor would quickly surpass Zimmerman in competency, knowledge, and personal/social skills. I can imagine a jealous Zimmerman, refusing to think of the Doctor as a sentient being, but as his creation; and becoming increasingly annoyed that the Doctor, who is like Zimmerman (at least on the outside, and with regards to basic personality), surpassing him. In Greek mythology, for example, whenever a human being surpasses a god (or thinks that they can), they meet horrific consequences.
Honourable mentions: Gul Dukat and Kodos the Executioner, and Lore and Ruk (from TOS episode: What are little girls made of?).
Thanks for reading, and I apologize if any of my English was strange - English is not my first language.