Wow, it hurt, but I still have to say Oblivion was the worst, if for no other reason than ret-conning the seemingly interesting jungles, lake-countries, wide, sweeping wind-chisled plains, and beautiful river valleys of pre-Oblivion lore Cyrodiil into a boring generic sword and sorcery world. It would have been okay if Cyrodiil was boring according to lore, but it wasn't, and Bethesda missed a great chance to have terrain as varied, if not more varied, than the complex ecosystem of Vvardenfall.
I chose Tribunal as the worst expansion, even though I personally loved it, because it was just too linear, it's main quest didn't "feel" like an Elder Scrolls Quest, more like a well made Fable quest with Elder Scrolls Lore.