It would be almost impossible to ever designatr a sitting president as the best president the United States has ever had. Often, a president can not fully be evaluated until the full effects of their years in office have been felt. Obama has had some significant successes, but some failures as well. This is true of all presidents. How he will compare remains to be seen, but being designated the best ever would be a reach.
Anybody would be hard pressed to say that Barack Obama is the very best president in the history of the country. There have been so many excellent presidents over the years that it would be a hard fight. He has done exceptionally well considering the hurdles that have been placed in his way, but he could have been more active and less militarialistic.