Hi. I'm Anna. I've never posted on this board before either but I hope you won't mind me jumping in here.
Coincidentally, my oldest is also Meagan and almost 15.

She's really developed quite an attitude lately.

Though it sounds like it's nothing compared to what you're dealing with. So first I just want to say
Second, as far as the smoking goes...I'm sure you understand this better than me but if she's been smoking for a year already then at this point it's a serious addiction and she's not likely to be able to quit or even cut back without help. No matter how many punishments or restrictions you put on her, the cravings won't go away. Have you talked to her doctor about it? Again I'm sure you know this already so I hope I'm not annoying you... but this is a serious health issue. So if it were me, my first step would be to talk with her doctor and try to get her some help on that end.
As far as the defiance and attitude....I'm going to suggest something crazy here.

Maybe she needs more responsibility. Something specific that she can take ownership of....take some pride in for herself. Is she involved in sports or other extra curriculars? Is there some way she can get involved in community volunteering? She'll probably fight you on it at first but maybe seeing how others are much more worse off than her....seeing how blessed she is....maybe it might help???
I also agree with the pp. She's probably going through more in her life than you realize. Is there another adult in her life she can talk to? One of your friends maybe? You could also talk to her friends parents. If they're struggling with some of the same things then maybe you could share some ideas and help eachother out by having the same expectations.
That's all I got. I hope at least some of it helps. Good luck.