I think The Sun Makers is hugely under rated. It's up there with my favorite Robert Holmes stories and I think it makes the best use of K9 of any of the stories.

I love the way the rebels are not shown as being perfect and being rather blood thirsty. I am not sure The Doctor would have worked with them if he had been given a choice but he does try to temper them a bit. This adds a level of complexity.

Oh and I think you can tell this story was written by someone who was fighting with the tax people over how much he owed them in taxes when he wrote it.