What Chief is accountable for DevOps? CIO


  Chief information officer (CIO), chief digital information officer
  (CDIO) or information technology (IT) director, is a job title
  commonly given to the most senior executive in an enterprise
  responsible for the information technology and computer systems that
  support enterprise goals.


and


  CIOs manage a business’s IT systems and functions, creates and
  delivers strategies and policies, and places great emphasis on
  internal customers.


CTO


  A chief technology officer (CTO), sometimes known as a chief technical
  officer, is an executive-level position in a company or other entity
  whose occupation is focused on scientific and technological issues
  within an organization


and


  CTOs place emphasis on the external customers to the organization and
  focus on how different technology can make the company more
  profitable.


COO


  The COO is responsible for the daily operation of the company, and
  routinely reports to the highest ranking executive, usually the chief
  executive officer (CEO). The COO is usually the second in command at
  the firm, especially if the highest ranking executive is the Chairman
  and CEO.


Discussion

As DevOps is closing the gap between several disciplines, e.g. Development, Operations, Quality Assurance one could argue that the CIO is accountable for DevOps as all these parties are internal customers. However, as a CTO is also focused on the outside world, one could also say that the CTO is accountable as he or she could implement DevOps because he or she heard in the outside world that this discipline was beneficial for some companies.
From my experience (where I currently work and another company where a friend works at) the DevOps team's report to the CIO.

Also to note that our DevOps team works a lot with, mostly alongside the CTO on a "day to day" basis compared to the CIO.