I'm in so much trouble please help :O?

I have two groups of friends that don't like each other. All of both groups like me. Group A are really fun to hang around with like if your with them your always laughing. And even though i'm not completely part of there group I don't ever get left out. Group B contains my very best friend. The thing is in that group sometimes no one has anything to say a lot of tension is about especially now. Me and my best friend fell out because I felt like she was replacing me for another girl in the group. My best friend was really nice and said she'd never replace me and all that. I think it was more of the other girls fault really. Sometimes in there group i get left out but only since something that happened with my best friend. A few weeks ago the two groups had an argument. The other group told my best friend that she was to clingy with me. Ever since my best friends been being different when others are around. She finally said to me yesterday why because it was making me unhappy and she said it was cos' of what they said. They all hate her the other group :L According to group B group A are trying to take me. okay yes that's true. Yesterday group A gave me an ultimatum they basically said "Your either with us or your not" I wanna go wth them cos' its such fun :) But I don't wanna lose my best friend cos' when its just us two i have a load of fun too :) Today's gonna be so tough :( Also another reason not to go with group B is cos' the girl who wants to replace my best friend kinda is ignoring me. :( So i will get left out if i go with them and if i do i'll never have the other group to go to again. HELP ME!