I had to think about this one.. *thinks*
*spoilers*
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Derek and Mark's friendship seemed a little... Forced. I mean I don't know anyone in a real world who would be friends with their best friend after they sleep with their wife. He essentially broke up their happy home. Then he game to Seattle grace to try and steal her back again Lol.
But once he started dating Lexi he calmed down I guess.... But there aren't many friendships to dislike ?? Idk this one was my least favorite question so far.