I love it when the stuff suspended in the miso separates just right and you can see the little convection cells in action. Definitely the most interesting soup to watch (and tasty!)
Tangyuan Soup - watch the rice based tangyuan slowly melt into rest of the white soup
Tofu skin soup - watch the light dried tofu skin expand. Is especially impressive with a clear soup base
Egg and bean curd sweet soup (tang sui) - watch as the thin pieces of bean curds swirl around the much heavy and thicker boiled egg
Rice noodles soup (various) - watch as different ingredients seem to bounce unexpectedly, as the rice noodles are fully transparent when submerged
Tom Kha soup - watch as the green curry based soup emulsion slowly separates into a dazzling array of red chilli oil, dark green of herbs, and the light green of the soup base as it cools down