I'm partial to the double conspiracy theory
(that neocons blew a hole in the Pentagon but Israelis learned about the plan and destroyed the towers)
Discussion
Not just the the towers, building 7 as well. It seems too many people inside the US government and around it, including the owner of trade center benefited enormously from the whole thing. Would have been impossible to organize the demolition without the cooperation from US secret agencies and the owner of the WTC.