CIOs, who do you rely on most for driving change management?

That is one loaded question. In reality, a breadth of people. Immediately, I would say I rely on my direct reports when pushing change into the IT Organisation, as in this change, I am sponsoring and leading the change. This takes a considered approach, inclusive of clear active communications, several frameworks for communicating change (e.g. Create and Communicate Direction, ADKAR, etc) and a lot of coaching and support.

When leading externally to the IT group, rely upon Peer executives to assist in driving the organisation change/response to any change let from the IT Organisation.

Finally, in the case of projects/products, having an executive sponsor with active delegates to work with the IT teams is key. Methods to the external stakeholders rely on a principal-based approach, the first of which is Transparency, which again is achieved through active communications/presentations through to showcases and playbacks. reporting success continuously.

PPM, PMO or the CxO that's in charge of portfolio / project management. Change leadership, for instance, is another matter and more complex than change management.