What is the best tasting broth to poach eggs in?

Parmesan broth

It depends on how you are going to use the egg. If it is just going to served on toast or an English muffin plain water is fine. For something like a frisee aux lardons you might want to use something more savory like chicken stock. But keep in you will also want to work in some white vinegar to set the albumen which makes milk not such a good match. Really though, there is no "best".

Milk. Then place the lightly poached egg on toast. Strain the warm milk and pour it over the egg and toast.