Pretty easy for me, this one. Time and the Rani is a stand-out stinker from the original series of Doctor Who. Bad in so many ways, from dreadful writing, poor acting, lacklustre direction... there is little this story does well. Pip and Jane Baker are writers who have only written one story I liked, Mark of the Rani, and even that was pretty ropey in places.
From the new series of DW, End of Time stands out. So poorly executed by people who have shown they are capable of so much better that it's insulting to its audience. I liked David Tennant, but this story basically highlights every wrong decision they made with his character through the series and hyper-sensitises you to them, so when you see those minor elements appear while rewatching earlier stories, they piss you off where previously you weren't fussed.
The Bakers were always ordinary writers, but RTD is one of the best of his generation, and as such I hold him to a higher standard. EoT was poo.