Email. Async comms make sense 99% of the time at my job. Unless there's deep work to be done, or pie-in-the-sky idea fabricating. Or rubber-ducky sessions. But I won't do those with AI.
Email is Calm Technology[0] for collaborative knowledge work, where you expected to spend hours on a single task. If something needs brainstorming, or quick back and forth, you jump on a more synchronous type of conversation (IM, call, in person meeting).
[0]: https://en.wikipedia.org/wiki/Calm_technology