sincerely, it won't be easy and you prolly won't get it accepted right away but tell them what you did was wrong and how sorry you are. make it up to them (don't ask how can I make it up to you because then you imply that you don't know them very well) after you apologize offer to do something together that you know you both enjoy or better yet something that they enjoy but they know you don't it will show how much you really care for them.
well first of all you should tell your friends they cant make u choose between each other even if one doesnt like the other and then u shuld tell your friend sorry by simply saying "i'm really sorry i shouldnt have been so mean to you because another one of my friends doesnt like you, I'm sincerely sorry." Or somethin like tat
Well I would talk to the best friend and ask why exactly they hate your other friend. And try to compromise. I'm sure you could come to an agreement. And for the other friend i'd definatly apologize saying you weren't thinking of their feelings but tyour best friend's. Hope things go well, and hope this helped. :)