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!