Really unfair to mention Time and the Rani, because its easily the worst of them all. Whatever issues anyone can have about End of Time, its still well made enough, and Willfred is a lovely companion.
Not to mention, its really a post-regeneration story. Its Seventh's first story. Its like Castrovalva that opened with Tom Baker regenerating - tahts not a regeneration story, is it?
So I'm gonna ignore that option, and say that my least favorite of the rest of them is, really, the TV Movie. As both that, and as a post-regeneration story (rather unique in that regard), its rather lacking, because the Seventh Doctor doesn't really do anything. The only thing he's supposed to do is take care of the Master's remains, and he can't even do that. Plus, his death doesn't quite have the weight of his previous, or his later incarnations.
So even without having seen Tenth Planet and Planet of the Spiders, I'm going with that still.