Well, seeing as it was so cringeworthy bad that it drove me away from 'Doctor Who' for a few years, my vote has to go to 'The Horns of Nimon', whatever the guy at message 2 says!
In polls in the years immediately after it was shown it was always down as one of the worst of any 'Doctor Who' story, and I think was voted THE worst more often than not. However, that was before 'Time and the Rani' and 'Delta and the Bannermen' which, for me, are the worst two 'Doctor Who' stories ever by some way.
It was interesting to see that 'Nimon' was not in the bottom 5 DW stories in the recent poll somewhere or other and wasn't even down ad the worst TB if memory serves (Underworld may have got that 'accolade'), so views do change. I remember when 'Warriors of the Deep' first came out I thought it was the worst story since 'Nimon', but now just see it as a below average story.
Ignoring 'Nimon', my vote would probably go to 'The Invasion of Time', which was both boring and silly.