I guess if I'm out in public, it'll usually be legs crossed at the ankles. However at home, particularly at the computer I always feel the need to put my legs up, so I generally sit in a rather peculiar position where I lean back in my chair, with my feet up on the desk in front of me, ankles crossed right beside the monitor, which is maybe 30 degrees to the right of me. My keyboard is on my thighs (which are at the perfect height at this point) and my right arm stretches out to use the mouse on the desk.
Yeah, I'm gonna have such a messed up spine when I'm old *sigh*.