What LSB should I buy ?

none of them, just farm them, most likely any you add won't be played regularly, and new squads come along and become more useful... although if you had to choose one, I would suggest Aphra, one of my favourite squads (I farmed her as soon as she came out, was a bit of a grind as I am F2P, but i got there just as I will with Master Mace (after i finish GL Ashoka farm)