If the property market is in a slump, why have some real estate investment trusts, profiled below and on the following pages, doubled or tripled? Ask hedge-fund manager Bill Ackman of Pershing Square Capital Management, who has been buying real-estate stocks. He says they represent the best value in investing. The Down Jones Equity All REIT Total Return Index has risen 28% this year, twice that of the benchmark S&P 500 Index. That’s on top of a 33% advance in 2009.