The CEO- I agree with Daniel Newman. Digital transformation is a means to an end and certainly not just digitising current processes and workflows. It is part of the continual re-invention of the enterprise and public sector organisation.

The CEO leads and the other members of the C-Suite should form a formidable transformation team.

Data underpins the whole organisation; that which is accessible and analysable and that (often the majority) that is hidden ( despite document management systems)  in emails, letters, freeform text, speech, video and images, letters. Poor and incomplete data= poor and incomplete transformation.

So the CDO is a key ally and probably the Digital Transformation Project Leader. Ably supported by the CIO and CTO. Loose role definitions and "mission creep" make it more likely that these three roles intertwine.

Line of Business Leaders must be heard- the CMO representing the customer and Business Unit Leaders specific products and services

Now a word of warning. Expertise sits across the organisation and too often that mixture of common sense and practical wisdom is ignored. 

The Military is not always known for its capability to change - not for nothing the old adage "Generals always fight the last war!" So if you wanted to deliver a project on time, within budget and to spec it was always wise to go the sergeants who had to deploy new technologies rather than the officer class.

C-Suite leads but front-line operatives often have the insights and practical knowledge to test the strategic and tactical digital transformation plans.

So:-  leadership from the CEO is the answer but teamwork no less a critical success factor