6. Jason Teague
Jense Ackles, we love you! Unfortunately we can't say the same about Jason Teague.
Ackles joined the cast of Smallville in its fourth season - a move that sparked a two-decade run on The CW that would see him co-star in Supernatural and appear in spinoff The Winchesters. His run on the DC series, however, would last for just one season, and that all had to do with his awful character.
Jason arrived on the scene as Lana's secret boyfriend, which automatically made him an obstacle in the Clark/Lana romance. The biggest issue with that was the fact that he took a job as a teaching assistant at Smallville High when he was dating Lana, which was all kinds of wrong. He was also lying to her and found himself unable to accept her history with Clark; which often resulted in the two clashing.
Then there's the fact that he and his mother Genevieve has ulterior motives, and when everything went wrong, he threatened to harm Martha and Jonathan Kent (which is unforgivable at this point).
It's wild to think that Jason could have been a regular part of the show for a bit longer had Supernatural not taken Ackles to greener pastures (and better characters) because one season was enough for fans.