M. Stevens tights and unitards are the stretchiest ones I've found. They are a very thin nylon/lycra blend that contours well to your body.
If you are going to wear socks, you can avoid footed tights. Otherwise, you'll want the footed ones. One word of warning: if you buy footed tights, don't wander around without covering the feet with socks or shoes. They'll wear out instantly if you do![/i]