For me, there definately isn't a worst story. I have favorites, but I can't choose a worst one. I guess I'm like Russell T Davies in the fact that I see greatness in all of TOM's 178 episodes and all of Doctor Who's 769 episodes overall thus far. Even the ones that fans voted second last and last in the recent Doctor Who Magazine 2010 poll like 'Time And The Rani'(Sylvester McCoy) and 'The Twin Dilemma'(Colin Baker) I still enjoyed. TOM's episodes from Robot to Logopolis and now Hornet's Nest and Demon Quest(soon I will have a listen!
I have no doubts it will be a classic! ) have all been like a rollercoaster ride for me. They have been such FUN! Even in my least favorite stories, TOM managed to make them still enjoyable with his riveting performance. So I definately have favorites and least favorites, but none of them were bad. I LOVE every episode of TOM's. Trying to choose which one is the worst or which one is the best is like trying to choose between my left leg over my right or my right arm over my left. You need them all. They are all important, and there's not a worst one. It's all in the EYE's and EAR's of the beholder.
" 'Wibbsey' would be a wonderful word for pillow talk, wouldn't it? Especially if you said it softly..."
Doctor Who Magazine #425 (Interview)