I love knee high boots. I'm no fashion expert but I would stay away from anything above the knee (sorry, I know they're enjoying a moment but they just look totally slutty to me) and look for something with a narrow cut and no buckles, fringe, etc. for a classic look. Buckles seem to be in now in the US but I think it's best to keep it sleek and simple, anything too chunky can start looking goth, and fringe looks too Olsen twins circa 2006.