I think it is the opposite of that. YC companies are not really competing with each other for investment; the success of a few helps the others, too. That Dropbox is worth over a billion dollars probably raises the valuation of each of the YC companies in w2012 by some measurable amount. There are a lot of investors who would only invest in certain kinds of companies, too, so it isn't even possible to have a universal rank ordering of attractiveness to investors.