Which invention is the best reason why the ottomans were able to conquer the byzantine empire?

The ottoman empire did not defeat the Byzantine because it had invented any unique special weapon.

No, Historians agree that Byzantine fell and was conguered due to its own internal problems. There was rampant misrule, incompetence and diplomatic slips, as well as unfavourable taxation. In addition, the people were bitter with the rulers because of focus on military expenditure at the expense of the people. They thus welcomed the Ottoman with both hands, and most of them converted willingly.