My favourite is the third one.
Because the guitar is so large and she is quite small, the other positions to me feel not quite right and slightly awkward. This is my personal view and depends what you are aiming for.
When you have the guitarist as the main focus by using the angle that you did in the third it seems more natural. The line of the guitar also leads into the image, whereas in the others it is so dominant.
Don't stop doing these type of photos though, you have found some really nice light and, even better, you use it well.