The entire cast of ABC's Inhumans very nearly made this list, considering that both individually and as an ensemble, they categorically failed to bring the popular Marvel characters to life.
Most dispiriting of all is Iwan Rheon's performance as Black Bolt's brother Maximus. Rheon was clearly only cast here in a cynical attempt to siphon off the Game of Thrones fanbase, which is somewhat ironic considering how much of his dialogue sounds like first draft Thrones material.
Where's the enthusiasm? Rheon seems weirdly not-fussed for much of his screen time, like he's got his mind on more pressing real-life concerns. Like the rest of the cast, though, he's shackled to horrendous writing and a cheap production across the board.
Considering how his involvement sparked a lot of the initial interest, his work here is bad enough to even make Thrones seem like a semi-fluke (though it most certainly isn't).
At least with the show's low ratings and poor reviews, it doesn't seem likely he'll be called back for more. Just sweep it under the rug and move on.