1. Lana Lang (post-season 4)
I hate to put Lana Lang on this list, much less at the top of it, but unfortunately fans just did not respond well to the character after Smallville's high school years had come to an end - and so much of that was down to the creative team's struggles to find anything to do with her.
Lana was Clark's teenage love in the comics and the show adapted that well. The chemistry between Kristen Kreuk and Tom Welling was off the charts, and the pair provided the show with some of its strongest scenes. Unfortunately, the writers kept taking their relationship around in circles, refusing to allow Clark to tell Lana the truth about who he was. The lies eventually sent her into the arms of his best friend Lex Luthor and turned her into a toxic shell of who she once was.
It's hard to know where it all started to go wrong with Lana. Was it when she became a witch? Lex Luthor's wife? A Kryptonite-absorbing superhero? I don't know, but the decision to do all of those just highlights the lack of direction. The latter made for a beautiful-but-heartbreaking send-off for her, but many fans couldn't overlook the decision to make her marry Lex and lose all sense of who she was.
I am a huge fan of the Lana Lang character and thoroughly enjoyed Kristen Kreuk's performance, so I don't agree with all of the negativity directed towards the character after season 4. That being said, I do think the decision to have her marry Lex completely destroyed her, and it was very hard for her to come back from that.