I'm going to fourth Bianca and Babe--this thread was made for them. There is no possible way it could be worse. Game, Set, Match, McTrash truly broke the mold.
While this one ended ridiculously and took away all the good will Bianca had earned over the years so that Babe wouldn't go to prison for being the lying, cheating, felonious whore that she was, I still think
Greenlee/Kendall is the worst friendship, ever, because once they became "best friends" it was all about how Kendall could help Greenlee while Greenlee stood in judgment of Kendall.
Unlike Babe, who was a good friend to Bianca initially and at least showed the occasional bout of guilt over being a lying, cheating, felonious whore, once it was established that Greenlee was Kendall's "best friend" she exposed Kendall's faked pregnancy to get her committeed of a murder she didn't commit, she married Ryan to keep him from marrying Kendall, she convinced Kendall to go with her to steal Ryan's precious sperm so she could have his baby, when it was discovered that Greenlee is too toxic a human to give birth, and when they thought Ryan was dead, she allowed Kendall to stupidly "prove her love" by carrying one of the Ryan/Greenlee embryos, only to accuse Kendall of trying to have her own Ryan baby once it was revealed that Kendall used her own egg, fled town for years leaving both the now undead Ryan and Kendall to deal with her selfishness, only to return and kidnap the baby she rejected years earlier and get into an accident that nearly killed the baby.
Then...after they throw Greenlee down another hole of redemption, she suddenly is absolved of all sins and turns on Kendall because in a moment of grief over losing her husband and "best friend" stupidly has sex with the "best friends" boyfriend. So Greenlee turns that into a reason for her to torment Kendall for the next few months about how much she and Zach bonded in that hole and how well she knows Zach, and how extra super awesome she is because she's Greenlee.
And I'm sure I'm missing about 150,000 shitty things Greenlee did to Kendall in the name of friendship, but whenever Kendall attempted to retaliate berated Kendall for being a lousy person and friend.