ThunderGr: The ladders on IYT work the way you describe it. They don't use levels, though. It's just a numerical ranking. Not only can the top person be constantly challenged, but they can lose their place just by virtue of the #2 winning their own game. The ones on GT are similar to BK, but they're more diligent about contracting them regularly to close out the gaps (but you can only play those on the same level).