In your opinion, which function SHOULD own AI governance at your org?

I think this depends a little on the people/person(s) taking the key roles. All components of the functions listed are important and even critical to success, so you really do need someone with the experience, authority, trust, respect and charisma to drive value for your company. Where they sit exactly.... maybe not the critical piece of the puzzle. If I had to choose.... 'IT', relabeled to something more relevant for the new digital era.

AI will have both intended and unintended impacts on multiple functions across the enterprise. Although one area or another (e.g., IT or GRC) may have a more prominent role, the organizational epicenter may shift as new capabilities or risks emerge.  Active involvement across multiple functional areas, IMHO, has the best chance of both tapping the full potential of AI as it applies in the organization's context, as well as more broadly sharing knowledge, leading to more effective and applicable outcomes.