I agree that Juliet makes the most sense.
Sarah was pregnant in 2004 O, right? but the timing is wrong for David to be 12 in 2004 X. Plus, he doesn't even meet Sarah in the X timeline without a car crash involving Boone/Shannon's father, right? (I can't remember any more.)
Still trying to figure out how Jack and Kate wind up together in X. I guess she's the only friend Claire has in L.A. and Jack just met his sister. Maybe Kate will get legal represenation from Ilana or something..