All of the above?
They are all various means of body language, and I use them depending on how I feel and what I want to communicate.
Legs spread is confident, relaxed, even arrogant. It's a show of force and dominance to spread out your body and take up space.
Legs together--thighs, knees and feet--tends to be neutral, professional, alert. Interview legs! Among other things.
Crossing your legs really depends on how you do it. Legs crossed and feet tucked under is defensive, nervous... or just cold. You never know.

On the other hand, legs stretched all the way out and crossed at the ankles is the single most dominant posture a human can present. It's often how the boss (particularly men) will sit in a business meeting.
And there are endless variations and combinations. Knees together and feet spread is childish, vulnerable, cute. Etc.
Body language is fun!