Its like Castrovalva that opened with Tom Baker regenerating - tahts not a regeneration story, is it?
So even without having seen Tenth Planet and Planet of the Spiders, I'm going with that still.
Of course Castrovalva is a regeneration story. It's essentially the final third of a 3-part story arc that begins with The Keeper of Traken
(the return of the Master), continues in Logopolis
(the Fourth Doctor falls from the radio telescope and has to regenerate), and the story picks up less than a minute later in Castrovalva
, while the regeneration still hasn't finished (it takes the Fifth Doctor most of that story to settle in to his new body and psyche).
I'm sure that Planet of the Spiders was an emotional story for the fans who were used to Jon Pertwee as the Doctor. After all, up to that time he was the longest-running Doctor (5 years). My first experience of Doctor Who was the Tom Baker era, and when it came time bid him goodbye in Logopolis, I cried through the whole thing.
My choice for the worst, though, is also the Rani story.