There have been credible claims of a handful known to be out there with collectors, in particular Web of Fear 3 seems very likely, it would likely be a number in single digits if there are any. The problem here is that it says there are 97 missing (true), but then could be taken to mean all of these are missing because collectors have them. Rather they are all assumed to have been destroyed, but it is possible a small number escaped that fate and are still out there in the hands of collectors

City of Death for me. Rounding out the top 5 would be Talons Of Weng Chiang, Robots Of Death, Seeds Of Doom, Ark In Space.

Tales from the Security Kitchen - who doesn’t want to learn more about the Security Kitchen? For example, why is there a security kitchen?

No Complications - explores the back story of that one Ogron who speaks English much better than the others.

The Adventures of Belal - The doctor’s little sidekick from Death to the Daleks in a series of adventures involving various menacingly tiled floors.