To my opinion, to answer this question, on must have tried to perform all of the in order to have a personal experience. For me, financial DD and Tax DD are more familiar and manageable as it happens to work as CPA for more than 20 years and Head of international taxation in the last years. It is not the same of legal DD which requires particular multidisciplinary knowledge (from common law to corporate law and labor law) in order to review commercial agreements, license agreements, legal disputes, knowledge of case law etc. Close to legal DD you could put also IP as well as environmental DD due to their close relation on legal issues and legal approach required. IT DD could be difficult for someone without specific skills and knowledge but it is easier to be performed by specialist (e.g CISA certified people). Finally commercial due diligence should be the most intriguing and probably requires the most interdisciplinary approach from financial accounting to management accounting, marketing etc. So I would conclude that commercial DD should be the most difficult
