8. Empire of Death
This season of Doctor Who was full of mysteries and build-up, and the culmination of that theoretically came in "Empire of Death." The God of Death, Sutekh, had risen and was ending the universe thanks to his travels on the TARDIS. While this was a major threat, the episode failed to maintain any tension, because it killed everyone off in the first ten minutes, an action audiences knew was going to be undone.
Since there was no question whether Sutekh would be defeated or not, the real question was how they would manage it. The answer was that they would taunt him with the identity of Ruby's mother, only to literally hook him on a leash and drag him through the time vortex. This was wildly underwhelming, particularly since sending Sutekh into the time vortex is exactly what the Doctor did last time (which didn't work).
On top of defeating Sutekh, the episode focused on Ruby finding her mother. Her significance to Ruby was touching, along with the mother and daughter's reunion, but it was done in a way that made little sense. Her identity was unknown to Sutekh, lost to all of time and space, but easily found with a DNA test? She named her daughter by ominously pointing at a sign, with nobody else around? It felt like the narrative version of "made you look," a taunting of the audience that disrespects their love of the series.
While the concept could have been a great way to connect the previous versions of the show with its newest iteration, it just didn't land right. The mysteries were solved in ways that didn't make sense, and the villain was defeated so easily that his threat felt meaningless. The season repeatedly promised greatness, but "Empire of Death" refused to live up to its potential.