I am looking for data showing the longitude at which the various planets in the solar system reach their winter solstice. Where could I find this data?
You might find in-the-sky.org's Custom astronomical graph plotter by Dominic Ford helpful. For instance, for Jupiter, a start date of today, you can obtain the declination graph of the next 10 years for a given location (which you can change as well):
This does not solve your problem that you want to determine the longitude of maximal declination, but still.