Obama's greatest failing is purely political. He and the Democrats pulled America too far to the left, setting the stage for his successor.
Bush's presidency is defined by his failures in Afghanistan and Iraq.
Clinton had the good fortune to be president during the information revolution and its resultant economic boom. Domestically, Bill did some good with welfare reform and the like. But, his foreign policies were muddled, laying the foundation for the international disorder of the 21st Century.
I was alive for Carter, Reagan, and Bush 41. But I can't speak from first-hand knowledge about their presidencies.