The one where the Doctor chases a man over land, air and sea...
Times really have changed, haven't they? These days, when it comes to a Doctor's last ever story, all the stops are pulled out to make it as memorable, emotional and headline-grabbingly special as possible (well, usually - that doesn't really apply to The Time of the Doctor). It's a big, important thing that the writer does not want to mess up. But with the classic series, Doctors' last adventures were a little hit-and-miss, and certainly not as heralded and well-sculpted as they would become.
And as a Doctor's final adventure, Planet of the Spiders has so far failed to impress on almost every level. Two episodes in - that's a third of the entire story - and nothing seems to be happening with any narrative urgency. The first half of part 2 bumbles along trying not to be noticed, while the second half goes all out to be as action-packed, but as mind-numbingly dull, as possible. As a whole, it's a mess.