Hertzsprung-Russel diagram for Active Galactic Nuclei (HR4AGN): Using space and ground based ultraviolet, optical, and infrared images to find a correlation between the color and luminosity of gas emission and dust emission around supermassive black holes at the centers of galaxies.
I found it most interesting to see the scientific process in action. Scientists were questioning scientists, sometimes fairly aggressively.
NITARP has been the best experience I have ever had for my personal and professional development as an astronomy educator.
I think that the most surprising thing about it was how young everyone was! I absolutely did not expect that, maybe because the amateur community is so much older by comparison, and is mainly composed of retired people.